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02-09-2012, 05:23 PM   #1
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Effect of Lenses designed for FF vs APS-C sensors

I thought I fully understood the whole crop vs full frame sensor issue, but clearly don't. I understand that if you use a lens designed for 35mm film on a FF camera then use it on a APS-C camera, the images look different as the APS-C results in a 'cropped' image because it is a smaller portion of a FF area so you are in effect cropping the image as you take it.

Now to my confusion which may be the result of reading some erroneous information. I've seen web sites that say that if you use a lens designed for 35mm film on an APS-C camera it will give different results than if you use a lens designed for cameras with an APS-C camera. So I (erroneously) thought that if I had a 50mm film era lens and used it on my APS-C camera, the image would look like I'd used a 75mm lens designed for a digital camera. I finally did a test to clarify the issue and found that if I set my two APS-C camera zooms (a Pentax and a Tamron) at 50mm and take pictures, the result looks identical to one I then take on the same camera using an old Pentax SMA 50mm prime. That surprised me, but not so much as I thought about it.

After looking at a number of web sites with various graphics, it seems to me that no matter what type of lens you use, so long as you use them on the same camera you will get the same result. If the camera is an APS-C device, then all lenses set or designed to a given focal length will give the same field of view in the resulting images. However, if you do this on a FF camera, then there would be vignetting on the photos that were taken with lenses designed for an APS-C camera. Despite that, the portion of the image that does show up should match the size of the image resulting from the 35mm film type lens. So if you printed them both, the size of objects in each respective image would be identical.

Many sites get caught up in the crop issue and only seem to compare and discuss results from a FF vs an APS-C camera. All I care about is results I'll get on my APS-C camera. So it seems that so long as I use my APS-C camera, I will get the same results irrespective of the basic design of a lens (FF or APS-C) so long as the focal length is the same.

Have I finally got this right?

02-09-2012, 05:29 PM   #2
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A lens designed for APS-C may project an image on the sensor with an area covering an APS-C sensor's size. A full frame lens will cover MORE area. There is no effect image-wise between the lenses.

In essence, you have mostly the right idea.

Last edited by JinDesu; 02-09-2012 at 05:49 PM.
02-09-2012, 05:41 PM   #3
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Any lens of the same focal length will produce the same image on your camera no matter if it was designed for APS-C, full frame, medium format or large view cameras.

If you use an APS-C lens (designed for smaller sensors) on a full frame body, you may get some vignetting when wide open. On the other hand, if you use a full frame lens on APS-C camera, you are using only the center part of the lens that generally has better quality.
02-09-2012, 05:51 PM - 1 Like   #4
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All the responses so far are correct. In general, it should be noted that:
-Full frame lenses are generally bigger, heavier, and more expensive
-Full-frame lenses designed for film may exhibit more CA and PF on digital

However, using a full-frame lens on APS-C digital has one big advantage: you camera can only see the center (sweet spot) of the frame!


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02-09-2012, 05:54 PM   #5
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Well, about 66% of the center, but yes - using a FF lens on a APS-C will make the FF lens look better.
02-09-2012, 06:12 PM   #6
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QuoteOriginally posted by atnbirdie Quote
So it seems that so long as I use my APS-C camera, I will get the same results irrespective of the basic design of a lens (FF or APS-C) so long as the focal length is the same.
50mm is 50mm irregardless of what camera you put it on. The only time you will notice any difference is when you compare the same lens on both a aps-c and FF camera. As long as all you have is aps-c then what you see is what it is. Lenses designed for film may perform differently on digital, this is supposedly due to differences in coatings and in the way the light is manipulated to strike the sensor.
02-09-2012, 06:15 PM   #7
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QuoteOriginally posted by atnbirdie Quote
I finally did a test to clarify the issue and found that if I set my two APS-C camera zooms (a Pentax and a Tamron) at 50mm and take pictures, the result looks identical to one I then take on the same camera using an old Pentax SMA 50mm prime. That surprised me, but not so much as I thought about it.
Yes, because they all still use the same focal length measurements. So even though your 18-55mm zoom is designed for APSC, the 50mm marker on it is still the same as a standard 50mm lens. They all project the same image regardless, just less or more of it is captured by the different sized sensors, that's all. Which is your crop factor.

This wikipedia image illustrates it the simplest - same size image projected by the lens, just capturing a smaller segment of it. If both are captured at exactly the same resolution, eg 4000 pixels wide, it will give the illusion the smaller image is actually closer up or zoomed in, hence a 50mm on APSC being an 'equivalent' of 75mm on full frame. It's the same as cropping and enlarging in photoshop.



So your APSC camera is simply not capturing that extra image lying outside the APSC sensor size area which the old 50mm lens is projecting, but a full frame camera could.

A lens designed for APSC is just basically so they can make it smaller and lighter by using smaller diameter glass to only project the required smaller (but still same size image) coverage onto the smaller sensor area only. Which in turn means this lens will only project onto the middle of a full frame sensor instead of giving full coverage.
02-09-2012, 06:18 PM   #8
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The focal length is the focal length - a 50mm lens designed for a full frame camera when used on a 1.5X crop camera will give you the same angle of view as a 50mm lens designed for a 1.5X crop camera - that is, about the same angle of view as a 75mm lens on a full frame camera*.



* - However it will not give you the same depth of field (at a given aperture AND a given subject framing) as a 75mm lens used on a full frame camera would - DOF is dependent on actual focal length - perhaps some of the confusion stemming from what you've been reading relates to this?

02-09-2012, 06:57 PM   #9
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QuoteOriginally posted by walternewton Quote
* - However it will not give you the same depth of field (at a given aperture AND a given subject framing) as a 75mm lens used on a full frame camera would - DOF is dependent on actual focal length - perhaps some of the confusion stemming from what you've been reading relates to this?
Don't go there, it will get very confusing. We had a long discussion here https://www.pentaxforums.com/forums/photographic-technique/173516-aps-c-vs-fu...e-subject.html about that.

DOF perception depends on many things including magnification of the print and viewing distance.
02-09-2012, 07:30 PM   #10
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Well, I think understanding the effect that focal length, aperture, and camera-to subject distance have on depth of field (and how sensor size/format choice interacts with these variables) is basic knowledge all photographers should strive to get a handle on the fundamentals of, regardless of how confused they might be at first.
02-09-2012, 07:33 PM   #11
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OK. Got it now for sure. I just got messed up by reading bad info somewhere that mistakenly said if you want a 'normal' lens (50mm) FOV on an APS-C camera, and you used a lens geared for FF, you had to use a 35mm lens. That's the downside of the 'interweb'. LOL
Thanks.
02-09-2012, 07:46 PM   #12
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QuoteOriginally posted by atnbirdie Quote
OK. Got it now for sure. I just got messed up by reading bad info somewhere that mistakenly said if you want a 'normal' lens (50mm) FOV on an APS-C camera, and you used a lens geared for FF, you had to use a 35mm lens. That's the downside of the 'interweb'. LOL
Thanks.
Well...as I read it that's basically true.

If (all else being equal) you want the same ("normal") angle of view that a 50mm focal length lens gives on a full frame camera, you need to use a (50/1.5) = approx. 35mm focal length lens on that crop sensor camera.

You might check out the Field of View calculators here:

http://www.tawbaware.com/maxlyons/calc.htm

What we're saying is it doesn't matter if that 35mm lens is originally designed for a full frame camera or a crop sensor camera - it will give the same results on the crop sensor camera.

Last edited by walternewton; 02-09-2012 at 07:58 PM.
02-09-2012, 08:02 PM   #13
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walternewton, You say if I want a photo to look like it was taken with a 50mm lens, I need to use a 35mm film lens. But that is ONLY if you are comparing two cameras with two different size sensors. However, if we are only talking about a single camera, like my APS-C camera, if I want a normal view, I can use any 50mm lens I can lay my hands on and they will give me the same result.
02-09-2012, 08:19 PM   #14
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QuoteOriginally posted by atnbirdie Quote
walternewton, You say if I want a photo to look like it was taken with a 50mm lens, I need to use a 35mm film lens. But that is ONLY if you are comparing two cameras with two different size sensors.
If you want the same angle of view you would get from a 50mm lens used on a full frame/35mm film camera, you would need to use a ~35mm focal length lens on a 1.5X crop factor APS-C body.

QuoteOriginally posted by atnbirdie Quote
However, if we are only talking about a single camera, like my APS-C camera, if I want a normal view, I can use any 50mm lens I can lay my hands on and they will give me the same result.
On your APS-C body any two lenses with the same focal length will give you the same angle of view.
02-09-2012, 08:24 PM   #15
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Understood.
QuoteOriginally posted by walternewton Quote
If you want the same angle of view you would get from a 50mm lens used on a full frame/35mm film camera, you would need to use a ~35mm focal length lens on a 1.5X crop factor APS-C body.



On your APS-C body any two lenses with the same focal length will give you the same angle of view.
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