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02-12-2021, 09:05 AM   #1
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Full-frame vs. cropped lenses on APS-C camera

We can use full-frame lenses on APS-C cameras, but that sounds like an awful idea, since only the center of the lens would be used. The total number of "pixels" that the lens projects onto an APS-C sensor is going to be 1.5 times less. Let's say a full-frame lens has a resolution of 2000 lp/ph. On an APS-C sensor that would become 1333 lp/ph. That's in linear dimension. Over the area of the sensor, that would be a 2.25-fold decrease in resolution. That's enough to turn and awesome lens into trash.

I understand that lens "resolution" is not the same as resolution of the lens+camera, but a 2.25-fold decrease in resolution of one part of the system is going to have a dramatic effect on the final image.

I also appreciate that a cropped sensor would "benefit" from using only the center of the FF lens where resolution is better than at the border, however, this is not nearly enough to compensate for using only a fraction of the image projected by a FF-lens. Border resolution is typically 10% less than resolution in the center, while the sensor size is 33% different (in linear dimension).

What is your real-life experience of using FF-lenses on APS-C camera vs using an APS-C lens?

I see that SMC-FA 35mm f/2 has glowing reviews. How would it do on an APS-C camera relative to SMC DA 35mm F2.4?

This thread confirms this, but the author did not actually state what camera he was using.


Last edited by Pentaxxoid; 02-12-2021 at 09:11 AM.
02-12-2021, 09:20 AM   #2
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I claim no expertise in photography, I'm just someone who posts a lot

you can check out my equipment by going to my profile and look at " about me " section

https://www.pentaxforums.com/forums/members/103786-aslyfox/?simple=1#aboutme

[ BTW, my wife has been very nice to me and allowed me to get good equipment and cameras ]

you will find lenses from third party lenses, lenses designed in the film era, both those not in production and those currently in production, lenses designed for digital both full frame and asp-c sensors

I find all to be good to use on either sensor

each sensor has something to offer depending on my photographic goals
02-12-2021, 09:36 AM - 5 Likes   #3
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I'm afraid you are wrong - well actually I'm happy you are wrong, because I can use my favorite FF lenses without hesitation on FF and APS-C bodies.
What makes you believe a FF lens would have a different resolution depending on the sensor format? The capture with an APS-C sensor just records the "best" i.e. center portion of the image projected by the lens.
02-12-2021, 09:38 AM - 2 Likes   #4
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QuoteOriginally posted by aslyfox Quote
[ BTW, my wife has been very nice to me and allowed me to get good equipment and cameras ]
My wife is into photography and steals my favourite lenses for her own use. You have no idea how lucky you are.

Most lenses are sharpest in the centre and weakest on the edges, so the crop will use the sharpest part of the FF lens.

Canon EF 35mm f/1.4 USM L II on 24 MP APS-c



Canon EF 35mm f/1.4 USM L on 21 MP FF


I know these are not exactly the same lens, but it illustrates what I'm talking about.
Look at the FF edges compared to the APS-c wide open. The APS-c cuts off the worst part of the image. And the end results are very close in terms of utility.


For the most part FF lenses do very well on crop sensors. When you look at my DA*55 1.4 and DA*60-250, some crop sensor lenses also do very well on FF. It's more about the lens and the size of the image circle, than it is about what they were designed to go on. The DA*200 was never redesigned for APS-c. It went from being an FF lens in earlier incarnations, to being a crop lens, then back to being an FF lens, with very little modification in the various incarnations.

I you want the best edge to edge performance an that is worth more to you than the extra resolution, 1.4 (or even ƒ2) on APS-c is the superior image.

If you think about it, if I mounted the above lenses on my Q-S1, 1:2.3 sensor, only using the very centre of the lens, it would be by far the most consisted edge to edge, and possibly the sharpest image.

The edges on most FF wide angle lenses are extremely problematic wide open.


Last edited by normhead; 02-12-2021 at 09:54 AM.
02-12-2021, 09:39 AM - 3 Likes   #5
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QuoteOriginally posted by Pentaxxoid Quote
What is your real-life experience of using FF-lenses on APS-C camera vs using an APS-C lens?
I use FF lenses on APS-C all the time.

Your premise and logic are completely faulty.
02-12-2021, 09:51 AM   #6
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QuoteOriginally posted by volley Quote
What makes you believe a FF lens would have a different resolution depending on the sensor format?
Not resolution, but total pixels projected onto a sensor.
QuoteOriginally posted by volley Quote
The capture with an APS-C sensor just records the "best" i.e. center portion of the image projected by the lens.
The problem is there are fewer of those "best" pixels. Would you still insist the same if we used a FF-lens on a phone-size sensor?

---------- Post added 02-12-21 at 11:52 AM ----------

QuoteOriginally posted by jatrax Quote
Your premise and logic are completely faulty.
Would you care to elaborate which step in my logic is faulty?
02-12-2021, 09:54 AM - 4 Likes   #7
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QuoteOriginally posted by Pentaxxoid Quote
Would you care to elaborate which step in my logic is faulty?
All of it.
02-12-2021, 09:56 AM   #8
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QuoteOriginally posted by jatrax Quote
I use FF lenses on APS-C all the time.

Your premise and logic are completely faulty.
My guess is almost all Pentax users do, or have done in the past. For years Pentax only made APS-c bodies, even though they had a full stable of FF comparable lenses.

02-12-2021, 09:57 AM   #9
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QuoteOriginally posted by jatrax Quote
All of it.
OK, I will leave it at it.
02-12-2021, 09:58 AM - 1 Like   #10
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QuoteOriginally posted by Pentaxxoid Quote
Not resolution, but total pixels projected onto a sensor.

The problem is there are fewer of those "best" pixels. Would you still insist the same if we used a FF-lens on a phone-size sensor?

---------- Post added 02-12-21 at 11:52 AM ----------


Would you care to elaborate which step in my logic is faulty?
Reread my post. It's all laid out for you.

The edge at ƒ1.4 on the FF image is 1437 lw/ph on FF and 2398 lw/ph on APS-c.
1000 lw/ph (a measure of resolution) is a clearly visible difference that will be obvious to the causal observer. Forget the theory, go with practical results.

Last edited by normhead; 02-12-2021 at 10:05 AM.
02-12-2021, 09:58 AM   #11
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QuoteOriginally posted by Pentaxxoid Quote
We can use full-frame lenses on APS-C cameras, but that sounds like an awful idea, since only the center of the lens would be used. The total number of "pixels" that the lens projects onto an APS-C sensor is going to be 1.5 times less. Let's say a full-frame lens has a resolution of 2000 lp/ph. On an APS-C sensor that would become 1333 lp/ph. That's in linear dimension. Over the area of the sensor, that would be a 2.25-fold decrease in resolution. That's enough to turn and awesome lens into trash.

I understand that lens "resolution" is not the same as resolution of the lens+camera, but a 2.25-fold decrease in resolution of one part of the system is going to have a dramatic effect on the final image.

I also appreciate that a cropped sensor would "benefit" from using only the center of the FF lens where resolution is better than at the border, however, this is not nearly enough to compensate for using only a fraction of the image projected by a FF-lens. Border resolution is typically 10% less than resolution in the center, while the sensor size is 33% different (in linear dimension).

What is your real-life experience of using FF-lenses on APS-C camera vs using an APS-C lens?

I see that SMC-FA 35mm f/2 has glowing reviews. How would it do on an APS-C camera relative to SMC DA 35mm F2.4?

This thread confirms this, but the author did not actually state what camera he was using.
I use an Irix 150mm F2.8 Macro lens (ff) on my k-70 without getting noticeable vignetting, check the compatibility tool, linked below.

(Beta) Pentax Lens Compability Checker Tool - PentaxForums.com

Here is a link to some pics!_IMG0026 | SharkyCA | Flickr
02-12-2021, 10:14 AM   #12
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QuoteOriginally posted by Pentaxxoid Quote
Let's say a full-frame lens has a resolution of 2000 lp/ph. On an APS-C sensor that would become 1333 lp/ph.
You assume that 2000 lp/ph is the most that can be squeezed out of the lens. It's quite possible that with a sensor with smaller pixels you could achieve a much higher resolution. IN fact it's quite possible that a denser sensor in the APS-c camera will achieve more resolution from the same lens inside the crop area as the FF does using the whole image circle. My 24 MP APS_c sensor equals a 20 MPCanon 6d sensor using a similar lens. In lw/ph terms the advantage to FF is about 20-25% even though the FF sensor is 50% bigger. The law of diminishing returns rears it's ugly head.
02-12-2021, 10:27 AM   #13
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QuoteOriginally posted by normhead Quote
You assume that 2000 lp/ph is the most that can be squeezed out of the lens. It's quite possible that with a sensor with smaller pixels you could achieve a much higher resolution. IN fact it's quite possible that a denser sensor in the APS-c camera will achieve more resolution from the same lens inside the crop area as the FF does using the whole image circle. My 24 MP APS_c sensor equals a 20 MPCanon 6d sensor using a similar lens. In lw/ph terms the advantage to FF is about 20-25% even though the FF sensor is 50% bigger. The law of diminishing returns rears it's ugly head.
That's perfectly possible. However, a lens designed for the smaller sensor would have exact same advantage.
02-12-2021, 10:29 AM   #14
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QuoteOriginally posted by Pentaxxoid Quote
The problem is there are fewer of those "best" pixels. Would you still insist the same if we used a FF-lens on a phone-size sensor?
Or no sensor at all The lens has no idea what sensor is behind the lens ... why should the performance be affected??
You are simply "overthinking" the issue. But don't worry, at one point in time most of us have been there.
My advice is: Just enjoy shooting ... and rest assured that FF-lenses will work without quality issues on APS-C.
02-12-2021, 10:32 AM   #15
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QuoteOriginally posted by volley Quote
Or no sensor at all The lens has no idea what sensor is behind the lens ... why should the performance be affected??
Because of fewer pixels making it onto the print (or monitor).
QuoteOriginally posted by volley Quote
You are simply "overthinking" the issue.
I would rather call it "inconsequential exercise in logic while having too much time staying at home"
QuoteOriginally posted by volley Quote
But don't worry, at one point in time most of us have been there. My advice is: Just enjoy shooting ... and rest assured that FF-lenses will work without quality issues on APS-C.
I am not a worrying type In fact, MTF charts have no effect on my decision to buy or use a lens.
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