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Comparing FF to APS-C: What difference does the bigger sensor make?
Posted By: dosdan, 06-03-2011, 09:42 PM

It is often mentioned that a FF sensor has 2.3x as much area as APS-C. So it should be able to capture a lot more light than an APS-C sensor and be better in low-light situations. This has many people craving for FF because it will allow them to shoot in tougher conditions. But this "advantage" is not as usable as it first seems. Let's look deeper.

FF vs APS-C (1.5x CF) has a 1.2 stops theoretical SNR advantage: https://www.pentaxforums.com/forums/photography-articles/129754-comparison-sn...ml#post1347917

I'll compare the Nikon D7000 to the D700, but you could just as easily substitute the Pentax K-5 in this discussion.

Due to the superior technology in the D7000's sensor compared to the D700, the advantage in FF SNR at true ISO200 is 1 stop. See the relative difference in the ISO200 18% SNR in Table 1: https://www.pentaxforums.com/forums/photography-articles/129754-comparison-sn...ml#post1346567

However, if you compare the two formats at the same DOF & FOV, then the FF camera will have to use a 1.2 stops smaller aperture to match the FOV & DOF of the APS-C camera.

Consider the following setups:
  1. 50mm FL, f/1.4 on APS-C at 7.5m subject distance
  2. 50mm FL, f/1.4 on FF at 5m
  3. 75mm FL, f/2.1 on FF at 7.5m
First off, FOV. Due to the "magnification" (1.5x Crop Factor) caused by the smaller sensor in the APS-C camera, to get the same FOV you either have to get closer with the FF camera, if using the same FL, or at the same subject distance, use a higher FL.

So for equiv. FOV:
  1. 50mm FL on APS-C @ 7.5m = 3.6m wide FOV
  2. 50mm FL on FF @ 5m = 3.6m wide FOV
  3. 75mm FL on FF @ 7.5m = 3.6m wide FOV
See: Dimensional Field of View Calculator at http://www.tawbaware.com/maxlyons/calc.htm

Now include consideration of DOF into these equivalent FOV situations:
  1. 50mm FL, f/1.4 on APS-C @ 7.5m = 1.26m total DOF
  2. 50mm FL, f/1.4 on FF @ 5m = 0.83m total DOF. Stopped down to f/2.1 = 1.26m total DOF
  3. 75mm FL, f/2.1 on FF @ 7.5m = 1.26m total DOF

So you can see that, when you include equiv. FOV & DOF, the FF setup has to reduce the aperture to f/2.1 to match the f/1.4 lens on APS-C. This is approx. 1 stop. So you lose FF's sensitivity advantage when you maintain FOV & DOF equivalency.

The only ways you can use FF's greater sensitivity are if you can accept either a shallower DOF or a wider FOV (you need to move the 50mm FL, f/1.4 on FF @ 5m, back to 6.1m to obtain a 1.26m DOF, at which distance you get a FOV width of 4.39m, so your subject is now smaller), or a bit of both.

The topic of APS-C & FF equivalency is covered in great detail in Joseph James' essay on Equivalence

Dan.

Last edited by dosdan; 12-04-2011 at 02:47 AM.
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06-07-2011, 04:22 PM   #2
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This makes little sense. A larger sensor means better image quality, all other factors being equal. In fact, you say as much in your article on point'n'shoot cameras. Why is it the jump from a p'n's to APS-C is an improvement whereas the jump fro APS-C to FF is not? Contradiction.
06-07-2011, 04:38 PM   #3
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Whenever an APS-C-only! advocate discovers the concept of Equivalency, they usually get pretty excited about it at first, because they think they've run across something no FF shooter has ever considered before. In reality we deal with it - and use it to our advantage - every time we shoot.

Basically - if you want to mimic the aps-c DOF/FOV equivalent, you will need to stop down to do so, which does negate all (D700) or half (D3s) the high-ISO noise advantage.

In reality, there are very few occasions where you feel the need to mimic the aps-c DOF, though - so you are able to take advantage of the better ISO performance, and enjoy more DOF control at the same FOV (lens-dependent, of course.)

Another way to look at it is this - FF can always match the aps-c DOF, but aps-c can't always match the FF DOF, because there aren't always lenses that can open up that wide, etc.

Where aps-c really has true advantages are in 1) size, 2) price, and 3) telephoto applications, when pixel density is taken into account.



.
06-07-2011, 04:49 PM   #4
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DOF-wise, FF can take you place where APS-C cannot. That is the main reason I cover full-frame. That and the more 3D look you can get, though my knowledge of optics is not enough for me to explain where the 3D effect comes from.

Medium format would be even better to me, but it is currently out of my price range.

Sent from my EVO 4G with CM7

06-07-2011, 04:54 PM   #5
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The crop format is just a subset of the full frame. Fundamental optics know nothing of sensor size.

The equivalence relations imply differences but the exact same results can be had obtained by using the a Full Frame sensor, lenses, distances, aperture, etc, to take a photo; crop the photo to ASP-C size and you have exactly the same results as if you'd started with the crop sensor, lenses, distances, aperture,etc, to take a photo.

The only difference would be the data you discarded by cropping the ff photo. The photographic results obtainable from a crop sensor are a subset of those of the full frame sensor.

There is nothing a crop sensor camera can do that cannot be accomplished by cropping a full frame shot.
06-07-2011, 07:20 PM   #6
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QuoteOriginally posted by newarts Quote
There is nothing a crop sensor camera can do that cannot be accomplished by cropping a full frame shot.
Exactly so.
06-07-2011, 08:21 PM   #7
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The OP read like an advertisement for full frame which I imagine was not the intent. The more I read defences of APS-C the more I want a FF and wish Pentax would hurry up and release one.
06-08-2011, 03:23 AM   #8
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The article was not intended to argue that FF is better or worse than APS-C.

If you want very thin DOF (moreso than you can get now with a 50/F1.4 on APS-C), or want better LL performance and are prepared to accept the DOF & FOV changes compared to APS-C, go for FF.

Just don't move to FF thinking that it is as simple as "bigger sensor = less noise = better picture".

Dan.


Last edited by dosdan; 06-08-2011 at 03:33 AM.
06-08-2011, 03:28 AM - 1 Like   #9
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QuoteOriginally posted by newarts Quote
There is nothing a crop sensor camera can do that cannot be accomplished by cropping a full frame shot.
Except that the APS-C version is usually a cheaper & lighter way to get that result.

Oh course, there's nothing that a FF sensor can do than that you can't get by cropping MF.

Dan.
06-08-2011, 03:48 AM   #10
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QuoteOriginally posted by rparmar Quote
This makes little sense. A larger sensor means better image quality, all other factors being equal. In fact, you say as much in your article on point'n'shoot cameras. Why is it the jump from a p'n's to APS-C is an improvement whereas the jump fro APS-C to FF is not? Contradiction.
The P&S article looked at high-end "bridge" P&S with relatively largish sensors. These are not that much cheaper than a K-r & kit lens, but the improvement in SNR performance in the step up to APS-C is quite significant (2.4 stops in the case of moving from a G12 to a K-x).

The SNR improvement in the step up from APS-C to FF is not as impressive (1.2 stops), but the cost & weight difference of FF bodies and FF lenses is usually quite significant.


Dan.

Last edited by dosdan; 06-08-2011 at 04:18 AM.
06-08-2011, 07:31 AM   #11
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QuoteOriginally posted by dosdan Quote
The article was not intended to argue that FF is better or worse than APS-C.

If you want very thin DOF (moreso than you can get now with a 50/F1.4 on APS-C), or want better LL performance and are prepared to accept the DOF & FOV changes compared to APS-C, go for FF.
"...If you want very thin DOF (moreso than you can get now with a 50/F1.4 on APS-C)..."

"...are prepared to accept the DOF & FOV changes compared to APS-C,.."

Dan, these are not neutral statements in my view. They contain a subtext and suggest that there's little to be gained by more DOF control at equivalent FOV, and that you need to be 'prepared to accept' some vague drawback in a FOV/DOF combination.

In reality - in everyday shooting - these work out to be useful things for the photographer. You can always stop down to match the aps-c DOF, thus marching (or sometimes still beating) the ISO performance. You can't always go the other way.

Giving the f/1.4 lens example suggests that you gain an advantage most people wouldn't care about - shooting with less DOF than f/1.4 would bring. This is a common example given in apsc-only circles and it misrepresents the situation a bit.

A good common example would use an f/2.8 zoom, or even 200mm f/2.8, for example. A 28-75 f/2.8 becomes equivalent to (on aps-c) an 18-50 f/1.8 zoom with regards to FOV/DOF. This can be an advantage in a lot of situations. Also, a 200mm f/2.8 'becomes' a 135mm f/1.8 prime - a very fun/useful tool for midrange telephoto. You aren't required to shoot it wide-open, but you have the option, and stopping down to f/4 to mimic the same DOF you'd have at f/2.8 on aps-c gives you that extra bit of sharpness and contrast most primes reach at f/4 while still retaining that smaller DOF. An ordinary 50mm f/1.8 'becomes' an extraordinary 35mm f/1.2, a common 35 f/2 becomes a nearly-impossible 24mm f/1.3, etc, etc.

I don't know if it's necessary to give a lot of practical examples in this article, but the text of the article takes (IMO) a slightly misleading course. To make the article more useful to the general reader it should represent or allude to the typical shooting situations we find ourselves in.

QuoteQuote:
Just don't move to FF thinking that it is as simple as "bigger sensor = less noise = better picture".
A broad generalization, but as far as generalizations go... somewhat true, all things considered.



.

Last edited by jsherman999; 06-08-2011 at 01:48 PM.
06-08-2011, 07:59 AM   #12
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QuoteOriginally posted by dosdan Quote
Except that the APS-C version is usually a cheaper & lighter way to get that result.

Oh course, there's nothing that a FF sensor can do than that you can't get by cropping MF.

Dan.
Exactly; it is a playoff between cost and effectiveness.

Further, what is possible with a miniscule sensor P&S is a subset of the possibilities for a larger sensor.

I find it somewhat amazing how many photographic situations for which a P&S camera is "good enough".

There are some hard limits that are not included in the usual equivalence analyses, like light diffraction limits to pixel size and pixel count requirements. ... it would be worthwhile to include such factors.
06-08-2011, 11:09 AM   #13
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QuoteOriginally posted by dosdan Quote
If you want very thin DOF (moreso than you can get now with a 50/F1.4 on APS-C), or want better LL performance and are prepared to accept the DOF & FOV changes compared to APS-C, go for FF.
This statement is still misleading. With FF you give up nothing in the way of FOV. Your wide lenses give you more FOV and your telephoto lenses can still be cropped to yield exactly the same image as on a smaller sensor.


QuoteOriginally posted by dosdan Quote
Just don't move to FF thinking that it is as simple as "bigger sensor = less noise = better picture".
Except that this is exactly true, all else being equal.
06-08-2011, 11:10 AM   #14
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So far I have neglected to mention that I came across this article only because I have written my own on the subject: Equivalence of Camera Systems. I would be happy for readers to critique that article, as it is my attempt at making these thorny matters as simple as possible.
06-11-2011, 06:42 AM - 1 Like   #15
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Having just sold a Canon 5D2 with a bunch of L series and Zeiss lenses and replaced it with a K5 plus older Pentax primes and zooms, I feel like I can make some comparison. The differences are small if you only print 13x19 or smaller, but if you crop or blow up really large a full frame sensor wins hands down on sharpness. I used the 5D2 for weddings, now retired from that I felt it too big to really enjoy as a general use camera. The K5 works great for that and I do not regret the decision to go smaller. I liked the narrow depth of field the 5D2 with the 50mm 1.4 Zeiss and I cannot get the same with the K5, but using a Pentax M-series 50mm 1.4 is close. It would be the same if it were not for the magnification.
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