Meetering fixed aperature

<p>I just purchased a fixed aperature mirror lens to use with the K5. This is my first experience using a fixed aperature. The information provided by the lens maker seems to indicated that meeting should be set to shutter priority. I read elsewhere that the metering should be set to aperature priority. I could use some guidance on how to best meter with this lens on the K5.</p>

<p>Definitely <strong><em>aperture</em></strong> priority. You want the shutter speed to vary to match the light coming in.<br>

On some cameras, you may have to adjust the exposure compensation to get exposure exactly on, as sometimes the combination of the peculiar structure of the lens and the way the camera takes its exposure reading may produce overexposure. Be Sir Chimp-a-lot until you figure out what's up in your case.</p>

 

<p>The actual answer is Manual mode for most or all older "M" mounted lenses. Shutter priority is useless because in that mode, you change the aperture and in this case, the lens has a fixed (unchangeable) aperture setting. You can use Aperture Priority mode to adjust the shutter speed to match the correct exposure and this method will work.<br>

<br /> The issue is that Pentax cameras have a crippled mount and no exposure information is translated to the meter. In fact the camera doesn't even know a lens is attached. The camera will give you a rough idea of the correct exposure but can easily be +/- 2 stops. With any all manual lens on a Pentax, you will have to use your experience to get the right setting.</p>

<p>Use manual exposure, and look at the histogram to choose the right shutter speed. Metering lenses earlier than Pentax-A on a Pentax DSLR is dodgy.</p>

<p>Sorry,<br>

I came in from "unified" view, and wasn't thinking about the apparently even-more-peculiarities of Pentax metering.</p>

<p>Why the so call crippled mount would cause under or overexposure? The camera simply measure the amount of light coming thru the lens and set the shutter speed based on the ISO. I have used fixed aperture lenses and with no coupling at all without any problem with my old Pentax KX film camera. What's the difference with the DSLR?</p>

<p>On a fixed aperture lens, it's not as bad but the camera does not know a lens is even attached so has no idea of the aperture the lens is at.</p>

<p>With a normal lens (M42, K, M series) that has an aperture, the lens might be set to f5.6 for example but be a 50mm f1.4 max aperture lens. In order for the camera to meter correctly, it has to know what aperture you have chosen. The lens is always wide open until the shutter button is pressed and then stops down for a split second (shutter speed time) to (as in this example) f5.6 to take the shot as you wanted. The issue is, if the camera doesn't know there is a lens attached, then it only sees the wide open setting (or amount of light coming in) and doesn't even know what that aperture is actually set at. Even if you manually stop down the lens to the setting you want to shoot at, the meter reading is off because the camera still has no idea what that aperture is and can not make the needed exposure calculation correctly.</p>

<p>When a camera takes a meter reading, it sees the wide open setting and then has the selected f5.6 setting information. It calculates that if the wide open setting is a shutter speed of say 1/500th @f1.4 and the lens is set at f5.6, then it knows to cut the shutter speed by 4 stops to 1/30th to get the same exposure at the same ISO setting. To even make that calculation correctly, it has to know that the lens attached is an f1.4 lens. In the Pentax crippled mount, it doesn't know and might think it's an f2.8 lens and base the calculation from that.</p>

<p>So if the camera has no idea that there is a lens attached and no idea what the stopped down setting is on that lens, the meter just guesses an exposure based on the available light and a guess on what the max aperture of the lens might be (the starting point to calculate the stopped down exposure). Normally the image will be 2,3 or more stops off. So even with a fixed aperture lens, the camera doesn't know it is f8 or whatever. It could think, this lens is an f4 lens and I'll choose a shutter speed to match that aperture for the amount of light I see in this scene. Now your shot is 2 stops over exposed.</p>

<p>To meter correctly the camera doesn't have to know the max aperture or the set aperture of the lens. It needs to know how much light is coming thru and when it stops down to set aperture how many stop less light will be. </p>

<p>OK then if you are right, you explain the problem.</p>

<p>There should not be any problem at all. Just use the camera in Aperture priority or Manual. </p>

<p>I know moderators (I am one elsewhere) hate people being confrontational but BeBu, you don't know what you are talking about and don't understand even when it's explained. Clearly you've never used an older (pre-A series lens) on a Pentax DSLR. </p>

<p>Without the contacts on the lens (or a camera that can read the aperture lever setting) to transfer lens and exposure information, the metering DOES NOT WORK and this is by design. The advertising claim that any Pentax mount lens will work on a Pentax DSLR is a partial myth. It will connect/mount and take a photo but not "work" correctly. Most certainly not like a modern lens can. In fact you would be faster to use a hand held spot meter and manually stop the lens down to get correctly exposed images.</p>

<p>It's too bad because there are some stunning M42, K and M mount lenses around and they work better on a 40 year old Spotmatic than they do on a K-5. It's understandable that an M42 lens can't communicate with a modern camera but am M or K lens could transfer basic aperture info to work if Pentax wanted it to. </p>

<p>I won't preume to know exactly how the camera arrives at its exposure calculation, but I do know that it works.... more or less. I have a couple m42 lenses that I have used fairly regularly on the K10D and K5, I set to aperture priority, stop down to desired f-setting meter and shoot. The camera doesn't nail the exposure near as often as on a modern lens but I don't remember it ever being much more than a stop off. The lenses are a 300 super tak and smc 500, the camera does ask for a focal length when I put these on so I'd guess it does know a lens is attached, and you need to go into the menu and allow aperature ring ( I think, don't have camera with me at the moment) Not taking sides, just telling how I do it.</p>

<p>Yes, with manual focus only non-A lenses you must go into a custom function menu and check allow aperture ring use. Of course, since your mirror lens is fixed aperture, that will not actually apply as you use the lens, because the aperture cannot be adjusted using an aperture ring. Nonetheless, putting the camera into this operational mode is necessary. The information is in your owners' manual for aperture ring use.</p>

<p>I have not yet used an old manual lens on my K-5, but it is no doubt the same as my other Pentax DSLRs. Manual mode is probably best, and then metering by using the green button, which will set your shutter speed. As Peter and Robert have said, don't expect absolute accuracy. You may get some overexposure, but then again maybe not. Once you determine how much it is off, if any, it will then always be off by that much, since you aperture will not change.</p>

<p>Once you have determined if the exposure metering is off, and by how much, you might then be able to successfully shoot in the Av mode, adjusting the exposure difference by setting the +/- exposure comp button to the appropriate compensation. </p>

<p>Michael, I was of course being facetious abut "the camera has no idea a lens is attached". It of course does know but just can not communicate with a pre A series lens and thus you have to set the focal length. That setting of course does nothing to aid in exposure readings.</p>

<p>I've owned at least 20 different Takumars and K or M series lenses from 28mm all the way to the monster 500mm. These almost all render wonderful images if you have the time to nail the exposure.</p>

<p>One day in a fit of boredom, I took 3 mid length primes and a hand held spot meter. Put the camera on a tripod and had things set up in controlled light. The exercise was to see if the exposure readings would be at least linear with each lens while using the Green button.</p>

<p>I can definitively say no. A lens might underexpose by up to 2 stops when wide open and over expose by 2+ stops when completely closed down. A one stop aperture setting change does not guarantee a linear exposure, one stop shutter speed adjustment change in the resulting setting/image. In fact, it's completely unpredictable. I tried even using the preview button and manually stopping the lens down, assuming that if the lens was set to it's stopped down value of say f8, the meter, using the green button, could get a more accurate reading of available light. Nope, it doesn't. It's closer to the right reading but often underexposed by 1/2 to one full stop. The camera just can't meter these old lenses accurately because it does not know what lens is attached and what settings you are using. <br>

The mount has been crippled and that's the end of the story unfortunately. If Pentax were to take Nikon's lead and allow these K and M mount lenses communicate with the body, Pentaxians would be a much happier bunch. I can't see this impacting lens sales for modern AF gear one bit but would certainly make the brand and bodies be much more attractive. What would really happen is, Pentax shooters would still buy the modern lenses they normally would but also add a few (or a lot) of older lenses for fun shooting and use them more. Our photography would be richer for it as well. </p>

 

<p>Peter, I'm inclined to agree with you in the sense that Pentax screwed this all up. What I don't understand is why stop-down metering doesn't even yield good results -- if the lens is stopped down, isn't it measuring the light actually passing through the stopped down aperture? Why can't it calculate proper exposure from this?</p>

<p> </p>

<p>There in is the mystery. Like I mentioned above, if you stop the lens down and use the preview button, the lens is at the aperture chosen, the VF is dimmed and the metering sensor is getting the amount of light at those settings and yet it's still wrong. Almost as if it was programmed to be wrong so you'll buy a DA lens and put your good Taks on Ebay.......</p>

<p>I suspect the best digital bodies for pre-A Pentax lenses may be Canon. Sadly. I'd definitely pay $200 more for a de-crippled (for pre-A K-mount) body in the K-5 class with adequate stop-down metering for M42, and I'm probably not alone. I'll bet Pentax <em>could</em> do it for $25/body...but they won't.</p>

<p>I've had good luck with my old Pentax "M" 135mm f/3.5 manual focus lens used on a DSLR. I soon figured out which apertures under or over exposed, and by how much. Once I got that down, it was easy.</p>

<p>But my old PZ-1p film body, even though having fully electronic controls very similar to a Pentax K10, 20, K-7 or K-5, can simply be fitted with an old "M" lens and just by selecting an aperture on the ring will provide aperture priority operation immediately. Even if the camera is set to Program mode! Don't even have to dial into Av mode- it shifts itself and automatically sets metering to centerweighted as soon as the lens is put on the camera. It is an amazingly fast, flexible, and efficient design. I have always wondered why such an outstanding capability could not have been carried over to the Pentax DSLR line. </p>

 

<p>I've noticed the metering discrepancy as well: The 35/3.5 SuperTAK that Javier gave me needs about +1/3 at any given aperture but the 135/2.8 Tokina needs +1-1/3 even wide open. It's rather strange.<br>

I have found that shooting in Manual is definitely the way to go though; I find the meter jumps around too much in Av for my taste... It's rather difficult to be consistent that way and I spend too much time in LR trying to compensate.</p>

<p>Metering discrepancies against actual aperture are in fact caused by the focus screen.<br>

In our DSLRs, the metering sensor is located on the pentaprism, and thus meters what it can see through the focus screen.<br>

The problem is, modern focus screen are not mere ground glass (ahem! Should say "plastic" instead!) anymore, but also present a Fresnel lens on the other side (the one facing the mirror). You can notice those strange concentric circles if you look at the focus screen itself, or in really low lighting...<br>

This Fresnel lens is optimized for two things : lessen vignetting by "straightening" the light rays in the corners, and "normalize" perceived brightness between f/3.5 and f/5.6 (kit lenses standard aperture).<br>

Vignetting in a viewfinder occurs because, contrary to the film/sensor taking the final shot, the light still has to travel quite a bit behind the focus screen. And oblique rays would soon end up lost.<br>

And have you ever wondered why your kit lens does not darken while your rack the zoom in or out? You go from f/3.5 to f/5.6, more than one stop!<br>

Another telltale thing is that nearly no focus screen can show you a f/1.4 DoF. Try your DoF preview: between f/1.4 and f/2.4, you won't see any difference in brightness or DoF...<br>

And if you don't, well, the metering sensor cannot do better...<br>

So, it all boils down to the focus screen and its characteristics. The K10/20 were notoriously bad in stop-down metering, and if you search a little, you'll find that they are the only Pentax DSLRs using a xx-80 naming scheme for their focus screens. Replace it with a LL-60 screen (made for the *istD), and stop-down metering just becomes bearable again (but still with a hefty -1/+2 Ev variation!)<br>

Now, if you are after the perfect focus screen for stop-down lenses, well, just go for a Canon EE-S screen (named S-Type over at FocusingScreen.com). This screen has the twofold advantages of 1- <strong>being truly linear against aperture</strong> (no more metering discrepancies!), and 2- <strong>showing an accurate DoF down to f/1.6</strong> (easier to focus than a split screen!).<br>

That's right: no matter what aperture is used in stop-down metering, you'll get a truly linear reaction from your camera. Closing one stop will provoke a one-stop increase in ISO (or decrease in speed, well, you understood the principle!)...<br>

Tiny change, big benefits!</p>