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Depth of field calculations SLR vs DSLR

annefromleo 15 11 1 United Kingdom
6 Sep 2006 9:09AM

I'm using a canon 75-300 zoom from my old film slr on a 350d. I understand the reduced field of view of the small sensor effectively makes the lens behave as the equvalent of a 50-180 (roughly).

What I need to know is, using depth of field calculators, which focal length should I use, the actual focal length of the physical lens at the frame size set, or the digital equivalent?

Example: slr zoom at 75mm equates to the digital zoom at a setting of around 45mm in terms of field of view. In a DOF calculator do I use a fl of 75mm or 45mm? (I would assume 75mm).

A speedy reply would be appreciated!

Many thanks

deviant 17 3.1k 1 United Kingdom
6 Sep 2006 9:19AM
It's the sensors circle of confusion that alters DOF for same lens on different crop cameras this online DOF calculator will sort it for you.
BigCol 20 1.8k 1 Scotland
6 Sep 2006 9:21AM
I'd say your assumption is correct as although the field of view is equivalent to a longer lens (you've got your calculations round the wrong way) the actual focal length of the lens does not actually change.
The 350D has an APS-C size sensor which gives a lens multiplication factor of 1.6 times which means that your 75-300mm zoom gives you a field of view equivalent to a 120mm lens at the 75mm end and 480mm at the 300mm end.
This page from canon here might be useful to you.

BigCol 20 1.8k 1 Scotland
6 Sep 2006 9:23AM
I really should type quicker! Deviant's link makes interesting reading and the calculator is great.
annefromleo 15 11 1 United Kingdom
6 Sep 2006 9:50AM
Many thanks Deviant and BigCol

You have clarified the issue for me, and yes, when I walked away from the pc and looked through the lens I realised I had got the conversion calculation wrong!

I can now go off and shoot knowing what I am doing (allegedly)!

duncan clarke 17 350 United Kingdom
6 Sep 2006 2:11PM
Nobody used to believe me when I used to say crop cameras give a different DoF. As a result though, I did some practical experiments to show the effect (albeit exaggerated, as it would be in a 3x crop):

DOF Test image 1
DOF Test image 2
JohnHawthorne 15 1.7k 5 Scotland
6 Sep 2006 2:21PM
Don't quite know what you are trying to achieve with those tests Duncan. All they do is illustrate that a longer focal length yields less depth of field. This is true no matter what the camera crop factor is.
keithh 17 25.8k 33 Wallis And Futuna
6 Sep 2006 2:28PM
Look through a 100 Macro on a cropper and FF sensor and the extra DOF is clear to see - this so called advantage diminishes as the focal length widens until it's all but unnoticeable. It's also the reason why you see people asking how they get blurred backgrounds when they are seemingly working at apertures that should do just that.
BigCol 20 1.8k 1 Scotland
6 Sep 2006 2:31PM
duncan, as John rightly says, your tests were flawed in so much as they don't really compare like for like! (alright he didn't actually say that but I just have)
But in answer to John's point maybe if you look HERE where Canon have actually done a like for like comparison you'll see that although Duncans experiment was flawed his hypothesis was actually quite accurate. At least I think that's what it shows. It may just be a pretty photo of a rose!
In thinking about the images shown in the link above, in order to get a similar field of view in the full frame sensor as that in the cropped sensor they'd have needed to use a longer focal length lens. And when you use a longer focal length at the same aperture you'll get a shallower depth of field, am I wrong?
For my next trick watch me disappear up my own orifice!
I should probably just delete this so as not to appear stupid but what the hell!
deviant 17 3.1k 1 United Kingdom
6 Sep 2006 2:38PM
The maths for it is obscene but the concepts a simple one. The more parallell the light hitting the sensor the sharper it appears. So on sensor with a smaller Circle Of Confusion (Henceforth COC) the lights more parallel. So the front and rear zones of apparent sharpness from point of focus increase. Simply put Depth of Field is down to COC size and lens length.
Carabosse 18 41.5k 270 England
6 Sep 2006 2:42PM
You can affect DoF by taking a pair of scissors and cutting around a print. Smile

.........OK - pull that one to pieces! Grin
Geoffphoto 15 13.5k United Kingdom
6 Sep 2006 2:46PM
I calculate the depth of field by how much my wellies sink into the mud at the football/rugby pitch every Saturday afternoon.
keithh 17 25.8k 33 Wallis And Futuna
6 Sep 2006 3:02PM
No CB...that's what is known as cropping.

If the concept or the maths are beyond people then it's best to let this one pass you by. Those who understand will either compensate or use it to their advantage. Those who don't are probably incapable of seeing, appreciating or otherwise the differences involved...they may not even care...I don't particularly care what causes the earth to spin...but it does regardless whether I know or care.
duncan clarke 17 350 United Kingdom
6 Sep 2006 3:05PM
Refering to my 2 experiments:

1) Proves what everyone already knows - if you use a longer lens you get less Dof. It does highlight however that in orderto take the same image with a crop cam as a FF cam, you do have to change the focal length. This is the same as the Canon example. Please note that though they don't say, the 2 images in their example were either taken at different focal lengths or were scaled differently.

2) Proves that the more you enlarge an image (from it's capture medium to viewing medium) the less DoF you get. If you produce a 12x8 print from a 300D, and the same print from a 5D keeping everything else exactly the same (aperture & focal length) then the 5D image will have more DoF.

The effect of (2) is a lot less than the effect of (1) but is not insignificant.

Quote:You can affect DoF by taking a pair of scissors and cutting around a print. Smile

.........OK - pull that one to pieces! Grin

This is only true if this results in you viewing the image from a closer distance. The size of the print has no effect on the DoF, but the amount is was enlarged and the viewing distance most definitely do.

In fact, I can change the DoF of an image without even touching it. I simply walk further away and ZING - more DoF. It's true.
Carabosse 18 41.5k 270 England
6 Sep 2006 3:06PM

Quote:No CB...that's what is known as cropping.

I know what it is known as, but are you arguing it doesn't affect the perceived DoF?

OK, let's put it another way. Cut around a print and then enlarge the cut-out to the original size of the print. DoF affected or not?

The point here is that DoF does not merely depend on focal length and sensor size.

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