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Auto ISO in A Mode and Focal Length?


mike_halliwell

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Asking for a friend, no really! I wasn't absolutely sure when she asked, so here goes...

 

She's a curious newbie and has got a Nikon 18-300mm 3.5/5.6 VR mounted on a d5300.

 

When it's in A mode and auto ISO, does it vary the shutter speed AND the ISO to make the exposure depending on the theoretical handholdable shutterspeed (1/focal length x 1.5 ish), ie at 18mm it might chose 1/50th whereas at 300mm it might chose 1/500th and use the ISO to make up the difference?

 

What are the inbuilt decision making factors?

 

I don't have any high ratio Nikon variable aperture lenses (although suspect this aspect will make no difference) and certainly nothing in the superzoom range to try it out... so thought I'd ask;)

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great question!

 

FWIW: I've tested quite a few Canon DSLRs and each seems to be programmed to vary the Tv (shutter speed) according to FL when the camera is in P Mode (no Auto ISO engaged). . . but I have never thought about what algorithms that might be at play when AUTO ISO is engaged.

 

WW

Edited by William Michael
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On my Df I found that it follows the 1/focal length rule. You can change that to a higher or lower shutter speed. This is consistent on the way down as the light getting lower. However I just discovered in the past week (I am not sure I have to test) that on the way up that is when the light is getting brighter it doesn't lower the ISO until the shutter speed get to max speed.
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The only thing I can find in the D5300 manual is the "ISO sensitivity settings" option under the shooting menu. On the D850 (which has a more searchable manual) this includes "auto ISO sensitivity control", which, if you turn it on, gives you options about minimum shutter speed (on the D850), including "auto", which is the focal-length dependent thing, and which has a faster/slower shift available. Without a D5300 in front of me, I've no idea how much of this is present on that body, and the manual doesn't help.

 

On the D850 (and D810, and I presume other recent bodies), "auto ISO" combined with auto shutter speed means you get a focal length-dependent shutter speed, although I don't know the formula, and the shift applies to it - which sounds like what you want. I'd have to try my D850 to check what formula it appears to be using. The thing that's put me off using it is that the "shift" to the shutter speed is only accessible by going through lots of menus, whereas if it were on a direct control I'd be able to adjust dynamically to the degree of support I have when shooting.

 

I suspect the only effect of variable aperture lenses will be that the largest aperture you can set will change with the zoom. The D5300 won't have the ability to keep the aperture constant in DoF preview while you zoom, unlike the D8x0 series, though.

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On my Df I found that it follows the 1/focal length rule. You can change that to a higher or lower shutter speed. This is consistent on the way down as the light getting lower. However I just discovered in the past week (I am not sure I have to test) that on the way up that is when the light is getting brighter it doesn't lower the ISO until the shutter speed get to max speed.

 

That doesn't sound right. While I've not shot in this mode for a while, it should preferentially reduce the shutter speed to the minimum, then start raising ISO.

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That doesn't sound right. While I've not shot in this mode for a while, it should preferentially reduce the shutter speed to the minimum, then start raising ISO.

 

It reduces the shutter speed to 1/focal length (it knows what focal length the zoom is set at) and then starting to raise the ISO. That I know for sure.

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On the D3200 there is an "auto" setting for the minimum shutter speed, which does the focal length calculation and sets the minimum shutter speed accordingly.. Trying this on a D3200 with a variable aperture zoom, I find that this calculation is made with a variable-aperture zoom, though I'm not sure how finely it is sliced. If you have Auto ISO on, the shutter speed will go only to the minimum set and then ISO will be raised, in that order. If you have Auto ISO set to the "auto" setting, that minimum will change when you zoom. Shutter speed will always go to the minimum first before ISO is raised.

 

Going in the other direction (I'm not sure why one ever would, but I suppose one might by accident), as BeBu says, if you set the ISO above the minimum, and shoot at a bright subject, shutter speed still takes priority. The camera will assume your chosen ISO is the default, and raise shutter speed as far as it can go, and only if it runs out of shutter speeds will it change ISO.

 

In either case, as far as I've ever seen, A mode will always take priority and attempt to adjust shutter speed first, and only if it runs out of shutter speeds will auto ISO take over.

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It reduces the shutter speed to 1/focal length (it knows what focal length the zoom is set at) and then starting to raise the ISO. That I know for sure.

 

Did a quick and dirty test with my D7100 and 3.5-5.6 DX 18-55 kitlens

Camera behaves as described above, when using A(perture) mode in combination with Auto ISO it dials down the shutter speed to 1/focal lenght and then raises the ISO depending on the amount of light (in the case of the 18-55 it chose 1/60th, probably due to the slow max aperture)

I believe more modern bodies (then the D7100 and D5300) take the focal length used into account when calculating the shutterspeeds if Auto ISO is used with A mode,

but don't have a more modern one around to verify that

 

That said. while I use Auto ISO a lot, I always set my camera on M (Manual) exposure

That way I always have control of both Aperture and shutterspeed, unike A (Aperture) mode where I control the aperture (and DoF) but risk getting a too low shutterspeed

in case there's too little light

 

Sure, if it's 'dark' I risk ending up using higher ISO values, but I still prefer a slightly more noisy sharp image over a low noise but due to camera shake blurry one

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Likewise I'm normally in manual mode and auto ISO, because of the inconvenience of shifting the auto shutter speed in auto ISO. (Manual is good if nothing surprising happens!)

 

I think I'm on the same page as BeBu, I was just confused by the "minimum shutter speed" terminology.

 

So, that said, does the D3200 let you "shift" the auto ISO automatic shutter speed like you can on the D8x0? Or, more importantly for the thread, does the D5300?

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Great answers everyone. Thank You!

 

I'll pass it all on...and of course I've learnt stuff too...:)

 

I've only ever used Auto ISO when in Manual and find it invaluable when i know what other 2 variables I want.

 

I'd noticed these 'ranges' when choosing Auto ISO and never really paid them much heed.

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Nikon used to issue a nomograph of what happened in Program mode with changing focal length/light level. However it was practically indecypherable and they seem to have dropped it from their manuals.

 

But in short, yes, Program and Aperture-priority modes do track the focal length with the shutter speed. (Is 'A' mode referring to fully automatic or aperture priority? Not that it makes much difference.)

 

Anyhow, a quick squint through the viewfinder while zooming in dim light will quickly give your friend the answer Mike. You can see the shutter speed increasing with focal length.

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I think I've managed to read it too (though, for odd EVs, it's suggesting that both shutter and aperture might change by half a stop, I think, which could be clearer; I guess I should try it). For the D850 it's in the menu guide for some reason.

 

I'm not aware of either program mode of itself tracking focal length (other than, possibly, with changes to a variable aperture lens's maximum aperture). Aperture priority certainly doesn't do anything with focal length. However, auto ISO will automatically increase the ISO if the selected shutter speed would have dropped below the length-dependent minimum, which has the effect of clamping shutter speed in both cases.

 

I think I should double check experimentally, though.

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Just had a secondary thought...;)

 

In the cameras/lens bodies that 'know' (or can be set) if they're in tripod mode does it ignore the 1/FL rule?

 

And yes A was indeed meant to mean Aperture priority, not Auto.

 

Is it the chip in the lens's job to tell the camera the FL it's on? Only reason I ask is I was relating this thread to a friend who upgraded the firmware on his D7100 and the AF died on his Sigma 18-50mm OS. Unfortunately, he was the guest at a wedding so went to MF.

 

However, when he reviewed them afterwards said that he was getting some crazy ISOs when using A with Auto ISO, kinda ISO 3200 at 1/1000 sec on a bright sunny day outside.

 

I wonder if the FL 'transmission' data was screwed up too?

 

He's going to find some RAW images with EXIF intact for me to have a butchers.

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I don't believe there's a mechanical means of transmitting the focal length to the camera (there's the focal length post, but that's fixed and only affects program shift on a few bodies in a binary way), so the focal length has to be a digital thing. It's been too long since I looked at the protocol to know more - but you do have to set focal length for a manual lens, which suggests it's a digital thing. (I'm not sure if the lens actually identifies itself, or just states focal length and aperture - DxO has been confused by the EXIF as to which 70-200 f/2.8 it's looking at, for example.)
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There's not much to understand about the "1/f" rule, used to set the minimum shutter speed in Auto ISO mode. In addition to manual mode, there may be a means to alter the effect of this algorithm to meet your needs. For example, in my Sony A7 you can set the minimum to "Normal, Faster or Slower". Don't recall if that option existed in my Nikon D3. I never used Auto ISO with Nikon, because there wasn't enough to go around.

 

Image stabilization changes the whole environment. The "1/f" rule alone reduces the effective resolution of your camera to 6 MP, due to camera shake. Image stabilization improve the effectiveness of ?1/f" by up to 5 stops.

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I don't believe there's a mechanical means of transmitting the focal length to the camera (there's the focal length post, but that's fixed and only affects program shift on a few bodies in a binary way), so the focal length has to be a digital thing. It's been too long since I looked at the protocol to know more - but you do have to set focal length for a manual lens, which suggests it's a digital thing. (I'm not sure if the lens actually identifies itself, or just states focal length and aperture - DxO has been confused by the EXIF as to which 70-200 f/2.8 it's looking at, for example.)

I think there may be some source of focal length coupling mechanically used by the FA and F4 but I don't think any modern camera use it. The focal length is readable by the camera for lens with CPU only. For the AI/AI-S lenses it uses whatever value you enter in the lens data and it can only be 1 value even if you use the zoom.

I use auto ISO in A mode because that does have the effect of clamping the shutter speed at the 1/focal length when the light goes low. So I normally set the minimum ISO at 100 which is on the ISO dial of the Df. By the default the minimum speed is 1/focal length but you can change it to higher or lower by some number of stops. I shoot mostly at f/8 or f/5.6. I use f/8 most of the time as I never want shallow DOF and stopping down more than f/8 I lose some sharpness unless I do need the extra DOF. I use f/5.6 when I want to keep the ISO a stop lower in low light condition.

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We're not talking about using ancient lenses here though, are we?

 

All that's needed to answer the question, as I said, is to watch the shutter speed while zooming in dim light. It shortens with increasing focal length, or at least on my D7200 with a G VR zoom it does.

 

This is the opposite of what you'd expect if the lens's variable aperture was responsible for changing the shutter speed.

 

It appears that the camera will vary the ISO to maintain a maximum 1/60th shutter speed at short focal lengths. As the lens is zoomed longer, the shutter speed shortens, with the Auto ISO increasing.

 

Obviously there's going to be a limit where the ISO just can't increase further, but in any sensible amount of light this doesn't happen.

Edited by rodeo_joe|1
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It doesn't use the 1/60 but rather 1/focal length. When I zoom out to 24mm the minimum speed is 1/25. Also on most new lenses the aperture would be constant if it's not set at max or min. For example when I set my lens at f/8 or f/5.6 it never changes although the lens is f/3.5-4.5 variable aperture.
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I think there may be some source of focal length coupling mechanically used by the FA and F4 but I don't think any modern camera use it.

 

Now I'm back at a computer, yes: there's a "focal length indexing lever" on the FA (at least), on the right hand side looking into the throat of the mount, and a corresponding ridge on some lenses longer that 135mm and teleconverters, which shoves the FA into a higher shutter speed program mode. I'b absolutely sure that no dSLR has an appropriate coupling (although I'd still like an "anything mount" camera that did), and I'd be very surprised if any zoom lens moves that ridge, although I guess they could.

 

The focal length is readable by the camera for lens with CPU only. For the AI/AI-S lenses it uses whatever value you enter in the lens data and it can only be 1 value even if you use the zoom.

 

True, although you can always assign multiple "lenses" to one zoom (and possibly toggle between them using the programmable buttons) if you really need to care. I still haven't checked, but I suspect the FA's length-dependent program mode behaviour hasn't been replicated in dSLRs, so the only difference would be for the matrix meter (in a way I don't entirely understand, but if it's a neural net, probably nobody else does either). The chart in the back of the D850 menu guide doesn't say anything about program mode shifting, but then it uses the example of a prime, so it wouldn't really come up. I believe the "increase shutter speed with focal length" behaviour that Joe mentions is because of the auto-ISO behaviour rather than innate program mode behaviour (so program mode won't select a wider aperture and a shorter shutter speed at longer focal lengths with auto-ISO turned off or not set to a focal-length-dependent minimum shutter speed) - but I could be wrong.

 

I use auto ISO in A mode because that does have the effect of clamping the shutter speed at the 1/focal length when the light goes low. So I normally set the minimum ISO at 100 which is on the ISO dial of the Df. By the default the minimum speed is 1/focal length but you can change it to higher or lower by some number of stops. I shoot mostly at f/8 or f/5.6. I use f/8 most of the time as I never want shallow DOF and stopping down more than f/8 I lose some sharpness unless I do need the extra DOF. I use f/5.6 when I want to keep the ISO a stop lower in low light condition.

 

Yes; I believe the same behaviour is in the other Nikon bodies (and you can, of course, also set a single explicit auto-ISO slowest shutter speed without the auto focal length thing): the direct ISO control (via a button on most bodies, via the dial on the Df) sets the minimum shutter speed, and the menu lets you change the maximum shutter speed - which always seemed backwards to me. The thing that puts me off using aperture priority (perhaps unfairly) is that so much menu diving is required to apply the shift to the focal length. For what it's worth, I've now been through the forum searching for anything that resembled a reference to my Nikon ideas list and have all the items in one place - but it'll take a while to write up and turn into a survey. Watch this space (if you care).

 

What I'm not clear on, for the D5300 that's the topic in this thread, is whether the auto-focal-length-shift thing is available on that body - and the manual doesn't say. I'll be mildly interested to know whether it's throughout Nikon's range.

 

Don't recall if that option existed in my Nikon D3. I never used Auto ISO with Nikon, because there wasn't enough to go around.

 

I believe the D3 (and D700, of which I had more experience) had an explicit slowest shutter speed for auto-ISO, but no "auto with focal length" option, which (for me) made it useless with a zoom lens. I doubt I was alone, but I requested auto with focal length, with a "program shift", when I was still a D700 user - so there's a tiny chance it was in the D800 because I asked for it (I also requested being able to reach the ISO button with my right hand, but that took a firmware update...) This makes me a bit embarrassed that I'm not really using it! One last tweak, please Nikon. :-)

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Andrews,

I always let the camera does 1/focal length so I never dive into the menu for that. I have problem with auto ISO is whenever I don't want to use the same exposure as the meter suggest I turn the camera to manual and I have to turn auto ISO off too. I never use the EC and normally I switch to manual it's no problem but I hate it when I have to turn off the auto ISO too.

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"What I'm not clear on, for the D5300 that's the topic in this thread, is whether the auto-focal-length-shift thing is available on that body..."

 

-I would be surprised if Nikon made their lower-end bodies less foolproof than their high-end ones. It wouldn't make sense to put off beginners by having a body that let them easily get camera shake in one of the auto-everything modes.

 

Besides, surely it would actually cost Nikon more to cut features out of their firmware, just to artificially cripple a lower-priced camera?

 

Hmmm! OK. It appears that implementing a sensible Program or AP mode algorithm is a recent move by Nikon.

 

For the first time I can remember, I put my D800 into P mode and Auto ISO. The lens focal length appears to have bugger-all affect on the shutter speed - not with a zoom nor by inputting different focal lengths into the non-CPU lens menu. In short; Program mode is total crap!

 

The D7200, OTOH, behaves very sensibly and shortens the shutter speed as focal length is increased. In both P and A modes.

 

A most strange and incomprehensible discrepancy..... unless my D800 is broken in some subtle way. Or some arcane 'Behave Stupidly' menu option has been accidentally activated.

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