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Crop factor and DOF


Krakman 14 3.6k Scotland
8 Apr 2007 11:20PM
What it really means is that when you use a crop sensor, you end up with less 'apparent' depth of field than if you're using the same lens with a full frame sensor (or film).

Of course, the ACTUAL depth of field hasn't changed at a given aperture (it's the same lens at the same aperture after all!), but because you've effectively 'zoomed in' on a distant object, you have amplified the 'defect' ie. the lack of depth of field, so the eye sees it more clearly.

Furthermore, an image taken with a 50mm lens on a 1.5x crop sensor will look pretty much the same, both in terms of angle of framing and depth of field, as an image taken with a 75mm lens on a full frame sensor.
User_Removed 16 4.9k England
8 Apr 2007 11:31PM
I know you're correct...... I read it in about 1952, when I studied these things. Then immediately began to forget, because I never needed to use them.
Lenses and crops present the image, the way they do and nothing can change it.
Col
theorderingone 17 2.4k
8 Apr 2007 11:54PM
Hmmm. that's right, in a way. A 50mm lens will always display the same DOF characteristics, no matter what the format is at a given aperture and subject distance.

A smaller sensor will generally be less forgiving of any mis-focussing, poor resolution or other defects as the pixel pitch will be smaller.

Crop factor does nothing to focal length, it only affects the angle of view, which has little or no effect on the DOF.

The get the same angle of view on FF as APS-C, you need a longer focal length if distances are kept the same. This in turn leads to a reduction in DOF due to the higher magnification of the lens.

In this photo for example, I took the same scene with a D200 at 50mm (same angle of view) and the water droplets were still in focus, whereas in this example (taken at 75mm, same aperture, same subject distance) the droplets are slightly soft. I'll post the D200 version if you like?

If you think of it in terms of magnification, the image cast on a crop sensor is smaller, so the magnification is lower for a given angle of view, so the DOF is slightly greater.

I've only ever seen a noticeable difference at brighter apertures (f/4 and brighter) and at closer distances, so the difference is slight for most people.

That's why I inferred that it's important for me before. 90% of my shots are taken at f/2 or brighter. For most general photography, the difference need not be worried about, so we can enjoy taking pics as normal! Smile

Horses for courses, as they say. Smile

I think we're getting somewhere now. Wink

P.S. My drink was ace, although I lost 6-0 at pool. Ah well.
Just Jas 19 26.3k 1 England
9 Apr 2007 12:47AM
That's why a camera, such as the S602Z, with a very small sensor and short focal length lens (7.8mm at the wide end)produces great DOF even at f2.8.

At the time when the camera was popular there were threads on here on how to try to overcome it, and produce a shallower DOF.

One reason why I like the S60Z still, is that most of my pics require large DOF.

Or the A610 which I am currently using.

jas
theorderingone 17 2.4k
9 Apr 2007 12:51AM
Exactly.

I found my S602Z great for product shots due to the large DOF.

I had to sell it in the end though.
Krakman 14 3.6k Scotland
9 Apr 2007 2:32AM
Hang on, now you are suggesting that a small sensor means more DoF? And we were so close to agreeing, too!

A 7.8mm lens (like the one on your S60Z) used with full frame sensor will appear for practical purposes to have almost infinite DoF, and it will still apppear to have a lot of DoF even when used with a crop sensor, so it's not surprising your S60Z seemed to have plenty of depth of field.

But for the reasons mentioned a smaller sensor will actually reduce the apparent depth of field compared to a full frame sensor on the same lens. If a smaller sensor gave you greater depth of field then taken to extremis you could take a standard full frame lens, and use a tiny sensor to get the same angle of view as, say, a 500mm telephoto lens, and get massive depth of field out of it. Unfortunately it doesn't work like that - it would be very nice indeed if it did! You would get the same depth of field as if you used a 500mm lens with a full frame sensor.

As already mentioned, reducing the sensor size does not alter the characteristics of the lens. It's the same thing as making a print and cutting out the centre area of the image with a pair of scissors. The characteristics of the lens remain exactly the same, as does the DoF characteristics of that same lens at a given aperture. But when you've cut out the centre of the image with your scissors, or your crop sensor, the DoF appears to be REDUCED, not increased, because of the simple fact that you're looking more closely at that part of the image!

In fact the relative sharpness hasn't changed (same lens, samme aperture, all that has changed is the pair of scissors!) but the way that you look at the print has changed. There is no optical magic to the crop sensor - it works in exactly the same way compared to a full frame sensor as the pair of scissors on the print...

Gotta go to bed now!
Just Jas 19 26.3k 1 England
9 Apr 2007 2:44AM
Whoa! I am not saying that the small sensor gives you greater DOF but that the associated small focal length lens (7.8mm - which gives the same angle of view as a 35mm lens on full frame) gives more DOF on the S602Z than a 35mm lens on full frame.

Because the focal length is smaller!

The matter was discussed at some length on here and various links to various learned bods websites were given at the time, which went into the physics and mathes of it.

I think we are broadly in agreement.

Sweet dreams!

jas
Carabosse 18 41.5k 270 England
9 Apr 2007 3:21AM
Why does this subject ALWAYS lead to a lengthy thread, and much hoo-hah?

If you want shallow DOF use a longish lens and a wide aperture. If you want extensive DOF use a shortish lens and a small aperture.

You will need to adjust your position from the subject - obviously.
theorderingone 17 2.4k
9 Apr 2007 5:00AM

Quote:Hang on, now you are suggesting that a small sensor means more DoF? And we were so close to agreeing, too!


If you'd read what I'd wrote carefully, or any of those articles you'd linked to, you see we are now actually in agreement.

It was your misinterpretation of the LL article that caused all this confusion in the first place. It's easily done if you just read the headlines and scan over the pictures. I had to take a second look before I realised what they were trying to say. Smile


Quote:
Why does this subject ALWAYS lead to a lengthy thread, and much hoo-hah?



Maybe because with all the different sensor sizes, formats and lenses available, it can be a difficult subject for some to grasp at first.., hence why it keeps cropping up!

(See what I did there!) Grin
User_Removed 16 4.9k England
9 Apr 2007 10:31AM
Throw in the word 'perceived'. The ratio of print to sensor size. Could go on a bit longer. Wink)
Just Jas 19 26.3k 1 England
9 Apr 2007 11:39AM
We could go on discussing this all night!

But as I had a late night last night (3.30am) I am switching to sleep mode.

Alright.....! More so than usual! :-]
dougv 17 8.4k 3 England
9 Apr 2007 11:50AM
Feeling deflated Jas?

You must have a PUNture...

Goan!

;o)
Just Jas 19 26.3k 1 England
9 Apr 2007 11:54AM

Quote:You must have a PUNture...



Yes, I am feeling a little 'tyred'! Smile)

jas
Krakman 14 3.6k Scotland
9 Apr 2007 12:20PM

Quote:It was your misinterpretation of the LL article that caused all this confusion in the first place.

Ahem, that guy's sloppy testing procedure didn't actually affect any of the analysis in the thread one jot...

But never mind, I'm glad we're finally in agreement (I think!)
certx 14 415 1 United States
9 Apr 2007 12:32PM
WOOOO HOOOOOO... you guys agreed.... I THINK Smile but, now I'm confused. Did we decide that the crop factor doesn't affect DOF? or that it does?

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