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05-22-2011, 02:18 PM   #1
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crop factor and FF cameras?

So i understand the multiply by 1.5 if you don't have a full frame camera...but I don't understand the difference between the quality of the image.

If you have a camera that has a 1.5 crop factor and you are shooting with a 50 mm, it would be the equivalent of shooting with a 75mm lens on a Full Frame camera right? But is it really equivalent? What I mean is....does it compress as much as a 75mm lens actually would on a full frame camera? Or is it compressing just as much as a 50mm would, but just cutting off the edges?

05-22-2011, 02:31 PM   #2
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QuoteOriginally posted by mhaws Quote
If you have a camera that has a 1.5 crop factor and you are shooting with a 50 mm, it would be the equivalent of shooting with a 75mm lens on a Full Frame camera right? But is it really equivalent? What I mean is....does it compress as much as a 75mm lens actually would on a full frame camera? Or is it compressing just as much as a 50mm would, but just cutting off the edges?
Quality doesnt suffer, its just the cropping that comes into picture. Think of it as a 1.5x of optical zoom which is fixed for APS-C. Optical zoom doesnt reduce the image quality. It just decreases the FOV. Simple Physics
05-22-2011, 03:02 PM   #3
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Ok, so....
If you take a picture of this tree with a ff camera, the whole tree takes up the frame, then using the same lens but use a aps-c camera you take a picture and only part of the tree takes up the frame....but you want the WHOLE tree to take up the frame, so you back up....then you depth of field is deeper, right? deeper than the full frame camera with the tree taking up the whole frame....so in a way...if you like shallow DOF, a FF camera is more capable. Right?
05-22-2011, 03:28 PM   #4
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QuoteOriginally posted by mhaws Quote
if you like shallow DOF, a FF camera is more capable. Right?
Correct

So thumb rule to get shallow DOF is
- Have Larger Sensor
- Shoot at Longer Focal length
- Have Large Aperture

Larger they are, more shallow your DOF is...

05-22-2011, 05:37 PM   #5
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The prior comments are all quite right. With a lens of any specific focal length, mounted on different frame-size cameras, you must position yourself nearer or further in order to fill the frame. Or if you stay in the same place, you need lenses of different focal lengths for each camera's frame-size. Changing distance changes perspective. Changing focal lengths changes DOF.

Cut out a picture from a magazine. On it, draw a rectangle that is 44x33mm -- that's about the size of a Pentax 645D frame. In the middle of it, draw a 36x24mm rectangle -- that's FF frame size. In the middle of that, draw a 24x18mm rectangle -- that's close to HF / APS-C frame size. And in the middle of THAT, draw a 18x12mm rectangle -- that's about m4/3 frame size.

Now suppose the picture was taken with a 50mm lens. The picture remains the same -- you haven't changed the image at all. But each smaller frame sees less of it -- or, each smaller frame crops more of it. That is ALL that crap.factor means, that smaller frames see less of a projected image. If you aren't a FF photographer making a transition to APS-C cameras, forget that you ever heard of crap.factor. A 50mm lens remains a 50mm lens, no matter what camera it is mounted on.
05-22-2011, 06:06 PM   #6
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It's a myth that longer focal lengths compress perspective. Focal length has nothing to do with it, perspective compression is purely a function of distance to subject.
05-22-2011, 06:44 PM   #7
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QuoteOriginally posted by twitch Quote
It's a myth that longer focal lengths compress perspective. Focal length has nothing to do with it, perspective compression is purely a function of distance to subject.
Thats correct, you cannot compress perspective. That is clearly shown in the diagram above. It will be clipped if the sensor is small as simple as that
05-22-2011, 06:49 PM   #8
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QuoteOriginally posted by mhaws Quote
So i understand the multiply by 1.5 if you don't have a full frame camera...
If you don't have a full frame camera, you don't multiply anything by 1.5. If you buy a 50mm lens, it is a 50mm lens, just like it says. Only if you also own a FF camera would you occasionally want to multiply the 50 by 1.5 to find what what lens you'd need to us on the FF camera to get the same field of view. But if you don't own an FF camera, there is no need to do that. That would be like driving down the road, seeing your speed in MPH on the speedometer, then constantly multiply that by 1.6 to find out what your speed is in KPH so you'd know if you were speeding in Germany. You're not in Germany, so why would you care?

QuoteQuote:
If you have a camera that has a 1.5 crop factor and you are shooting with a 50 mm, it would be the equivalent of shooting with a 75mm lens on a Full Frame camera right?
In terms of field of view, yes.

QuoteQuote:
What I mean is....does it compress as much as a 75mm lens actually would on a full frame camera?
As mentioned, no lens compresses a thing - that's a myth. It's your position relative to your subject that determines perspective. The reason this myth got started is that when using a longer lens, people tend to stand farther from their subjects, which changed the perspective, and some people erroneously assumed this meant the lens caused the change in perspective. But it didn't; it was the change in position. Since a 50mm lens on your camera has the same field of view as a 75mm lens on the camera you don't own and therefore really don't have any need to even be thinking about, you'll stand in the same position to take the picture as you would with the 75mm lens you don't own on the FF camera you don't own. But again, since you don't own that camera or that lens, why think about this?

And if you do insist on thinking about it, why stop at FF? Why not also constantly go around worrying about what focal length on a 645 camera would provide the same FOV? And also on 4/3, and also on 6x7, and 8x10? There's just no point in doing the math unless you actually own both cameras and are interested in buying a lens for one that matches the FOV of a lens you like on the other.

05-23-2011, 06:51 AM   #9
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I think it pays to ask the OP if he has ever used a 35mm film camera.
If he hasn't, this whole conversation is moot.
05-23-2011, 05:42 PM   #10
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I haven't owned a 35mm camera or a full frame camera. I am asking so that I understand what people are referring to when they talk about their cameras and lenses. I had heard full frame cameras were more professional and was trying to understand why. Thank you for helping me understand the difference better.
05-23-2011, 06:28 PM   #11
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QuoteOriginally posted by mhaws Quote
I haven't owned a 35mm camera or a full frame camera. I am asking so that I understand what people are referring to when they talk about their cameras and lenses. I had heard full frame cameras were more professional and was trying to understand why. Thank you for helping me understand the difference better.
35mm format DSLRs are certainly the high end "pro" models from the big three, whether they are "more professional" or not is as meaningless as wondering if a banana is more professional than a pomegranate. There are advantages and disadvantages to both formats, and in terms of professional viability, there is no way of saying that one has an advantage over the other.

Really, if you've never used a 35mm format camera, there is no reason to think about "crop factor".

However, it is a shorthand term for the difference in field of view between formats.
If you look at a 4x6 inch print and think of it as "full frame", then draw a rectangle on it 3 x 5 inches and you will get an idea of what the "crop factor" is in relation to 35mm vs. APS-C (Pentax digital).
Personally, I think it is better to learn and understand how things work on the format you are using than a format you are not using.
05-23-2011, 07:50 PM   #12
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And I will tell this story again:

Back in the day, my colleagues and I worked in various film formats, sometimes all on the same day: Miniature (135 Full-Frame; and Half-Frame, about the same size as APS-C); Medium Format (MF, mostly 6x6cm); and smallish Large Format (LF, just 9x12cm -- I didn't get into the larger 4x5" or 5x7" or 8x10" LF's). I could mount the same T2-base lenses on the 135/FF and 135/HF cameras; using bellows and adapters, I could also mount MF and LF lenses on those same Miniature cameras.

We did not talk of crap.factor, nor of equivalence. The marketing terms hadn't been invented yet. When I first mounted a 400mm lens on a 135/HF SLR, I thought, OH BOY! IT'S LIKE A 600mm LENS! But it wasn't; it was still 400mm, with the edges chopped off. We did not ponder the 'equivalence' of lenses in the various formats. We merely learned what focal lengths DID in each format. The diagonal of the frame marked a 'normal' lens. Twice that was tele; 2/3 that was 'wide'.

And when I shot (un)official portraits in ANY format, I learned to like 80-85mm lenses, often around f/3.5-4, often around the same distance (~1.5-2m). That combination of focal length and aperture and distance IMHO gives the nicest roundness to human features, without distortion. Each different format just captured a different crop of basically the same projected image, with the same DOF and perspective. Know what a specific lens does on a specific camera in specific conditions, and use it.
_________________________________________________________

Larger frames don't imply 'professional'. Working pros use whatever format is appropriate. Some of Galen Rowell's famed mountaineering pictures were shot with an Instamatic -- a VERY GOOD Instamatic, of course, the German Kodak (Nagel) 126-format Instamatic 500 with the beautiful Schneider-Kreutznach Xenar 38/2.7 (normal) lens. It fetches a good price now. But photojournalists have won prizes with images shot with Holgas and cruddy P&S's and whatever works. THAT is the hallmark of professionalism: Do whatever it takes to make it work, to produce the desired image.

A final note on digital FF vs APS-C: FF is a tiny sliver of the dSLR market, and dSLRs are a tiny sliver of the digicam market. MF is even more minuscule. Research money flows into areas with greater payoffs. More research bucks go into smaller sensors, P&S and APS-C, than into FF and MF sensors. And the payoff: smaller sensors just perform better per square cm. It's not like with film, where Tri-X has the same density in every format. Crop an FF image down to APS-C size and compare it with an equivalent APS-C image (same lens / settings / distance). Pixel-peeping WILL show a difference.
05-23-2011, 10:57 PM   #13
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QuoteOriginally posted by RioRico Quote
And when I shot (un)official portraits in ANY format, I learned to like 80-85mm lenses, often around f/3.5-4, often around the same distance (~1.5-2m). That combination of focal length and aperture and distance IMHO gives the nicest roundness to human features, without distortion. Each different format just captured a different crop of basically the same projected image, with the same DOF and perspective.
*All* lenses have the same perspective. Perspective is not a function of focal length or FOV - just of distance to subject. There is nothing special about 85mm lenses - it would have been pure coincidence that you happened to like that same focal length on different formats.
05-24-2011, 10:05 AM   #14
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QuoteOriginally posted by Wheatfield Quote
Really, if you've never used a 35mm format camera, there is no reason to think about "crop factor".
I guess the thing is that until you kind of understand it a little, you don't realize that it doesn't matter. And if you don't understand it and someone just says "it doesn't matter you don't need to know" then you feel kinda like well....why don't I need to know. lol Ironic.
05-24-2011, 11:17 AM   #15
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Yes, it's unfortunate that you do need to understand it in order to realize why it isn't normally relevant.

You are correct that it is *is* relevant when conversing with people who are talking about focal lengths in FF terms. But realize, that doesn't happen much at all on this forum, because Pentax does not offer an FF digital camera. So when anyone on these forums talks about using a particular focal length for a particular purpose, you would use the exact same focal length. Only if you drop in on the film sub-forum would have mentally have to convert everything to APS-C equivalent - so if they say they used a 50mm lens for something, you'd use something around 33mm to get that same FOV. Generally speaking, though, you'd never need or want to ever speak in FF terms if you're not using FF. Just say you're using a 35mm lens; people who shoot FF will know they need to convert that to 50mm in their own thinking. You don't need to do that for them; it will just cause confusion.
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