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APS-C Question


carl_s

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Hey All,

 

I am a Nikon full frame shooter and haven't owned an APS-C camera in quite a while. I'm advising a friend who is getting into photography, and now I'm starting to question my own understanding of the field of view differences between a full frame and aps-c body. Hoping someone here can straighten me out:

 

I know that in Nikon's case, FX lenses are designed for their full frame line. A 50mm prime yields a 50mm field of view on a full frame body. If you were to take that 50mm FX lens and throw it on a Nikon APS-C/DX camera, you would end up with a 75mm field of view because of the 3/2 multiplication factor that needs to get applied.

 

I had thought that the reason Nikon produced their DX lenses was that these lenses were made for their bodies with APS-C sensors. So a DX 50mm prime lens would give a 50mm field of view when mounted to an APS-C/DX camera body (Exactly the same field of view as an FX 50mm prime on a full frame body). The person I just talked to was telling me that regardless of which lens you put on an APS-C body, the 3/2 multiplication factor still needs to be applied?? Is this correct?

 

I also understand that if you take a DX lens and throw it on an FX body, the camera goes into crop sensor mode and throws out pixels, but I'm more interested in whether or not the multiplication factor ALWAYS needs to be applied when using an APS-C body, regardless of DX/FX lens choice.

 

Thanks in advance.

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Yes the multiplication factor needs to be applied. -DX / FX defines the image circle and doesn't change the focal length.

90mm is a superwide on 5x7", wide on 4x5", standard on 6x6cm and a portrait lens on FX.

 

The industry deserves some spanking for selling P&S digicams with 35mm/FX equivalent focal length descriptions and we should have gotten used to better i.e. more abstract definitions of our focal lengths in use long ago...

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If you were to take that 50mm FX lens and throw it on a Nikon APS-C/DX camera, you would end up with a 75mm field of view because of the 3/2 multiplication factor that needs to get applied.

That is correct.

I had thought that the reason Nikon produced their DX lenses was that these lenses were made for their bodies with APS-C sensors.

They are - their image circle is (in general) smaller than that of an FX lens and hence (usually) does not cover the FX sensor fully. Thus an APS-C-specific lens can be made smaller and lighter than an FX one of the same focal length.

So a DX 50mm prime lens would give a 50mm field of view when mounted to an APS-C/DX camera body (Exactly the same field of view as an FX 50mm prime on a full frame body).

Nope. Focal length is independent of the sensor size - a 50mm lens is 50mm on APS-C, FX, or whatever other sensor-size body you choose - what does change with the sensor size is the field of view (or alternatively, the angle of view). So a 50mm on FX is "normal"; on a medium format camera it would be a wide-angle and on a APS-C or m4/3 it becomes a light or medium tele, respectively. The commonly made mistake is to consider the "crop factor" to apply to the focal length when in reality, it applies to the field of view (or angle of view); the focal length of a lens does not change when you mount it on camera bodies with different sensor sizes.

The person I just talked to was telling me that regardless of which lens you put on an APS-C body, the 3/2 multiplication factor still needs to be applied??

Yes, if you want to know what the FX-equivalent field of view is - which in some cases is rather irrelevant.

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http://jdainis.com/tree4.jpg

 

A 50mm lens throws a 50mm wide circular image. In fig. A you see what falls on a full frame 35mm film/sensor. In fig. B you see what falls on APS-C sensor. That is what gives it a 75mm effect, the same would show on a 75mm lens on a 35mm film frame. Fig C shows what would occur with a 32mm lens on the APS-C camera. 32mm is a "normal" lens for the APS-C. 50mm is a "normal" lens for a 35mm or full frame camera.

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James G. Dainis
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As well as I know, Canon won't mount EF-S lenses on full frame cameras.

 

Some Nikon cameras adjust to use a smaller sensor area.

It might be that you can turn off that ability, and see how big the actual

image circle is.

 

For zoom lenses, the image circle usually changes with zoom, so it might fill FX at

some focal lengths, and not others.

 

Also, since teleconverters use the center of the lens image, and expand it to fill

the frame, a DX lens with TC should be able to fill an FX frame.

 

Yes, some simpler cameras give 35mm full-frame equivalent, but the actual

lens specification should give the actual focal length.

 

One article says that the focal length for the iPhone 3 camera is 3.85mm.

(Should be less than the thickness of the phone.)

-- glen

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As well as I know, Canon won't mount EF-S lenses on full frame cameras.

 

Some Nikon cameras adjust to use a smaller sensor area.

It might be that you can turn off that ability, and see how big the actual

image circle is.

 

For zoom lenses, the image circle usually changes with zoom, so it might fill FX at

some focal lengths, and not others.

 

Also, since teleconverters use the center of the lens image, and expand it to fill

the frame, a DX lens with TC should be able to fill an FX frame.

 

Yes, some simpler cameras give 35mm full-frame equivalent, but the actual

lens specification should give the actual focal length.

 

One article says that the focal length for the iPhone 3 camera is 3.85mm.

(Should be less than the thickness of the phone.)

 

I've used teleconverters on c-mount lenses to boost image circle, yes it works but not always as much as expected. TCs can have baffles to restrict light in areas the TC thinks are non image forming. If the TC is designed for FX it should enlarge the image circle enough but it might be very borderline with a 1.4x TC). A TC designed for DX might reduce the image circle of a FX lens!

 

3.85mm sounds reasonable for a phone lens. But there's no reason why it must be less than the phones thickness - Telephoto lens designs allow a lens to be physically shorter than it focal length - this is actually the technical definition of telephoto despite the fact the term has been misused to imply long focal length lenses (not all of which are telephoto in design).

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Does anyone have experience with the Sony E/FE lenses? I was thinking its the same as Nikons’s Dx/Fx, where an FE (full frame) lens will work just fine on an aps c body, but an E lens would need the aps-c.

 

But then when I talk to the fine folks at B&H they confirm that FE is for full frame, but also say an E lens works for both full frame and aps-c? This seems backwards from Nikons logic.

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I wish the words “crop factor” or “multiplication factor” would disappear from the jargon of photography. The crop factor only serves to confuse.

 

Historically, cameras were big because they needed to be, if you were making big prints. Then, the materials of the process evolved and big prints could be made using smaller cameras. Today’s miniature cameras can easily produce images that display well as big prints or on giant screen.

 

Because the 35mm full frame became so popular, this size became the de facto standard. Zillion of lenses were made and the market flooded with used but good lenses. As cameras shrunk, there was a needed an easy way to figure out what happens if FX lenses were mounted on these smaller bodies.

 

About lens focal lengths: Each format size is associated with a focal length that delivers a “normal” field of view. “Normal” means – approximating the human perspective. Such as lash-up is a focal length that approximately matches the diagonal (corner-to-corner) measure of the image frame. For the FX 24mm height by 36mm length, the diagonal = 43.27mm. Mount a 43.27mm lens of an FX and the angles of view will be 31.0° vertical – 45.2° horizontal – 53.1° diagonal.

 

Sorry to report that it’s the diagonal angle of view that is generally published. This makes about as much sense as TV sets being sold by their diagonal measure. This is the way of the advertising world.

 

The DX format measures 16mm height by 24mm length with a diagonal of 28.84mm. If you mount a 28mm on a DX, the angles of view are 31.0° vertical – 45.2° horizontal – 53.1° . Please note this is exactly the same as a 43mm mounted on a FX thus 43 ÷ 28 = 1.5 the crop factor.

 

What you really need to know: Mounting a lens with a focal length about equal to the diagonal measure yields what we call a “normal” angle of view. Makes no difference what size the image frame is.

 

Now a 43.27mm “normal” lens for the FX is sort of odd. The industry chose to round this value up to 50mm for convenience. Mount a 50mm on a FX and the angles of view delivered are 27.0° vertical by 39.6° horizontal and the diagonal is 46.8°.

 

What is wide-angle? This is a lens shorter than normal; the industry generally places this value at 70% or “normal” or shorter. For the FX, this is 50mm X 0.7 = 35mm or shorter. For the DX, this is 28mm X 0.7 = 20mm or shorter.

 

What is telephoto? By industry standards this is 2X “normal” or longer. Thus for the FX, 50mm X 2 = 100mm or longer. For the DX this is 28mm X 2 = 56mm or longer.

 

Why is knowing this stuff of any importance? As technology evolves, cameras shrink. Tomorrows cameras will be sub-miniature. I think the so-called crop factor will disappear. You will need this knowledge instead.

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Why a focal length about equal to the diagonal measure is labeled “normal”? Why for portraiture do we need a longer lens?

 

Factorial: If you look out a window at a vista, you can, with wax pencil, trace on the glass the outlines of objects. This trace replicates the human perspective. You can replicate this perspective with a camera. It matters not what size image frame or focal length lens. To replicate, make a contact print (un-enlarged print, same size as the image frame). Now view this print (or slide or computer screen) with your eye placed at a distance from the image equal to the focal length of the taking lens. When performed, your view of the image exactly matches the human perspective revealed in the wax pencil trace.

 

Sounds easy -- but wait; not easy when a miniature camera is in play. For this lash-up, you must make an enlarged print (enlargement print or enlarged display on a computer screen or TV.) Now the rules change. Say we used a 50mm lens on an FX body. To make an 8X10 inch display image, the miniature frame must be magnified a minimum of 8.5X. The viewing distance is now 50mm X 8.5 = 290mm = 11.4 inches. Factorial: The viewing distance to see an image as per the human perspective is the focal length multiplied by the magnification.

 

OK, say a 28mm is mounted on a DX, and an 8X10 inch print or TV is to be viewed. The magnification needed to make this displayed image is 12.75X. With this lash-up the viewing distance to see the image per the human perspective is 28 X 12.75 = 357mm = 14 inches.

 

Note: The viewing distance is about your normal reading distance.

 

Note: For most images, viewing per the human perspective is not very important. However, for portraiture and some other applications, this view is very important as it mitigates distortions. Make a portrait with too short a lens and the nose is enlarged and the ears reduced. The client says,” I don’t photograph well”. What focal length for portraiture for FX for DX. Work the math, say the 8X10 print is placed on a mantel or mounted on a wall. For this location, the viewing distance is likely about 1 yard. For the DX the ideal focal length is 105mm (105 X 8.5 = 890mm = about 36 inches. For the DX its 75mm (75 X 12.75 = 950mm = 36 inches (value rounded for convenience).

 

Moral of the story – Many make great portraits without knowing this; they likely gravitate to focal lengths that do the job based on their interpretation without knowing the science.

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Let us return to a time when photographers used large format.

 

http://jdainis.com/merlion4_5.jpg

This would be what a 4x5 inch Velvia positive film would look like if taken with a "normal" 150mm lens on a 4x5 film camera.

 

 

http://jdainis.com/merlion35mm.jpg

This would be what a 35mm Velvia positive film would look like if taken with a "normal" 50mm lens on a 35mm film camera.

 

Notice they both capture the same field of view (more or less). The 4x5 film could be laid on a piece of 4x5 paper to contact print the image directly. The 35mm film would have to be blown up in an enlarger to make the full 4x5 size print. Both prints would look about the same.

 

http://jdainis.com/merlioncrop.jpg

Remember, at this large format time the "normal" lens was 150mm. If one were to buy a miniature 35mm camera and use the"normal" 150mm lens on it (as one buys an APS-C camera and uses a "normal" 50mm lens on it) the 35mm camera would crop out the red square above and capture that on the 35mm film. Now people would say that the "normal" 150mm lens has a crop factor of 3x which causes it to become a 450mm lens.

James G. Dainis
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I wish the words “crop factor” or “multiplication factor” would disappear from the jargon of photography. The crop factor only serves to confuse.

+1 - a dozen years after the full frame sensors became mainstream and almost twenty years of "crop sensor cameras" and here we are still explaining the "crop factor".

This seems backwards from Nikons logic.

It's actually the same logic for Nikon and Sony: DX lenses can be mounted and used on an FX body - the camera automatically reverts to "crop mode" (that automatism can be turned off in the menu). Sony APS-C and FX cameras have the same mount: E-mount. Sony has taken to labelling FX lenses FE and using E for APS-C lenses only. Just like with Nikon, Sony FE lenses can be used on both APS-C and FX bodies and E lenses are meant to be used on APS-C bodies but can be mounted and used on FX bodies as well (either with vignetting or in crop mode). I am not going to comment on Canon - they have lately created a mess of what fits on what body.

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@ James Dainis - If an 8x10 print was made from a 4x5 film taken with a 150mm lens – the magnification to make an 8X10 is about 4X and the viewing distance for correct perspective is 150 X 4 = 600mm = 23 inches. The same vista, taken with a 35mm film camera with a 50mm mounted, requires an enlargement of 8.5X. Thus the viewing distance for correct perspective is 50 X 8.5 = 424mm = 17 inches. Both viewing distances are in the normal reading distance range. The perspective of both images will be about the same i.e. “normal”. Also, no hard fast rules as to what looks "normal", most photographs will look good even if the viewing distance is not spot on.
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(snip on TCs)

 

3.85mm sounds reasonable for a phone lens. But there's no reason why it must be less than the phones thickness - Telephoto lens designs allow a lens to be physically shorter than it focal length - this is actually the technical definition of telephoto despite the fact the term has been misused to imply long focal length lenses (not all of which are telephoto in design).

 

I did know that, but somehow didn't think about it at the time.

(Including the meaning of telephoto.)

 

Seems to me, though, that for phones the problem is getting wide enough lens, not for getting narrower lens.

 

But yes, phones could have multiple lenses with different focal length, and select an

appropriate one.

-- glen

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Thank you all. All great comments. Here's my next thought.... I understand that we are re-defining "normal" based on a target field of view (The 150 mm 4x5 example was a great one), and as sensors become smaller, so will the associated standard use focal lengths. With that said, as lenses move towards the ultra-wide end, is there any degradation in the optic? So for example, if I wanted a "traditional" full frame 24mm view and went with a 16mm focal length on an APS-C body, is the 16mm lens going to have any more distortion than the 24mm lens?
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Yes the multiplication factor needs to be applied. -DX / FX defines the image circle and doesn't change the focal length.

90mm is a superwide on 5x7", wide on 4x5", standard on 6x6cm and a portrait lens on FX.

 

The industry deserves some spanking for selling P&S digicams with 35mm/FX equivalent focal length descriptions and we should have gotten used to better i.e. more abstract definitions of our focal lengths in use long ago...

 

It's OK for labeling the P&S with 35mm equivalent focal length as the sensor size is generally not easily known. The spanking should go to those who use the 35mm equivalent on APS-C and 4/3.

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I was always told that the correct viewing distance of a photo/image was equal to the taking lens focal length. In the photo of the Singapore Merlion above, if i were to hold the 4x5 inch slide 150mm from my eye, it would exactly superimpose over the actual view. For the 35mm slide, I would have to hold the slide 50mm from my eye to have it superimpose directly over the actual scene.

 

If I had an 8x10 inch slide made in an 8x10 view, camera with a "normal" 300mm lens, I would hold it 300mm (12 inches) from my eye to superimpose it over the actual scene. I should think the same viewing distance would hold for the same scene blown up to 8x10 inches from 35mm or 4x5 inch view camera using the "normal" 50mm and 150mm lenses. All three scene captures should be the same assuming the 35mm is set for 4:5 capture.

James G. Dainis
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is the 16mm lens going to have any more distortion than the 24mm lens?

Depends? - While I agree with @AJG we might be comparing apples & oranges? - Leica made their 24mms for film, Fuji planned the 16mm end of their kit zoom for digital only. So yes, if I look at RAW and JPEG, side by side, I see a lot of distortion in the RAW, that gets digitally cleaned up for the JPEG. The result is IMHO "acceptable", especially considering that you get that zoom for a song and 3rd party heritage glass made for 35mm film bodies seems unlikely to ever outperform it. I haven't tested Fuji's primes but guess they perform better.

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For DSLRs designs descendant from 35mm film SLRs, the lens flange to film plane distance is fixed.

I suspect that it is a little harder to design a 16mm lens with the large flange distance for Nikon F mount

or Canon FD or EF mount.

 

But the newer mirrorless cameras from Canon, Nikon, and others, have a shorter flange to film

distance, and so should have comparable 16mm or shorter lenses.

 

There are many APS-C format zoom lenses that go down to 18mm.

That is much rarer for full frame lenses, but 24mm isn't so unusual.

-- glen

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