View Full Version : Depth of Field and the digital crop factor

02-05-2006, 02:51 AM
If I use a 50mm focal length lens from an old SLR on my *ist DS, will it give the same depth of field as a 75mm lens would have on a film SLR? (Assuming same aperture used etc.)

Sorry if that's a bit muddled - I hope you know what I mean?:o

02-05-2006, 04:27 AM
Basically it'll give you the same field of view as your 75mm on a film SLR, but the depth of field of a 50mm lens.
It's a crop, so the 50mm lens on the DSLR is basically the same image as a 50mm lens on an SLR, but you crop out the outside.

02-05-2006, 05:46 AM
thanks for that astro. So your 50mm f1.4 gives you a 75mm focal length (film equiv) when used on a DSLR, but gives you the same depth of field as it would have on a film SLR?

BTW thanks for thinking of me with regards to your 2nd 50mm, I have a bid on one on ebay at the moment :)

02-28-2006, 01:13 AM
First off, this is my first post. Hi everybody. I used to own a Pentax KM a few years ago. Wonderful camera, gotta love the needle exposure indicator. I've also owned a Canon Powershot A60 for a few years, which is slowly starting to die...and shutter lag is killing me!!!

So I ordered a Pentax *isd DL with the 18-55mm lens kit for ~$500! I know the DS has a bigger viewfinder, but they're quite a bit more expensive and seems lots of stores are out of stock so I just got the DL.

I'd like to clear up some confusion. It's been a while since my high school photography class, But from my understanding "depth of field" has more a relation to aperature setting than focal length. http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html

I also have a question. My question is related to this thread and also this thread: http://www.dcresource.com/forums/showthread.php?t=12177&highlight=focal+length+digital

From my understanding of the multiplying effect of digital Pentax cameras is 1.5X the focal length. I am in the same dillema as the referenced thread. I'd like to have a focal length that is more closer to human perception [50mm] but because of the 1.5x factor, does that mean in order to get that sort of perspective [meaning less distortion that a wider angle lens would give] I would have to purchase a more wider lens [say a 35mm, 35x1.5=52mm] to achieve this?

I like to shoot low light photos and would also like a bright/fast as possible lens. What would be the best lens for me to get? From my ebaying and googling 35mm. lens aren't that bright it seems [like 2.8 is the fastest] and they are very expensive. But I'd love to have a very fast/bright prime 50mm SLR equivalent lens! For now I'm just going to stick with the lens kit. Maybe it'll be better overall instead of changing lenses because of dust getting in the camera.

I really would've liked to have gotten an oldschool Pentax 50mm 1.4 or 1.7 lens but with the 1.5 factor this would giving me a more telephoto like perspective, correct?

Is there anything wrong with what I wrote? Please clear up my and this users confusion once and for all, wise forum participants. :)

02-28-2006, 02:22 AM
Basically it'll give you the same field of view as your 75mm on a film SLR, but the depth of field of a 50mm lens.
It's a crop, so the 50mm lens on the DSLR is basically the same image as a 50mm lens on an SLR, but you crop out the outside.
It is partly true, but also partly not true.

Depth of field is not a value, but a perception. Only one plane is in focus in a photo. In front and behind that plane things get gradually more out of focus. It is our perception that determines what we still see relatively in focus, and what we see as beginning to be blurry.

Now imagine the following (or try it out):
When you view a photo from a certain distance, you see a certain depth of field. What happens when you take a few steps closer? You see the out of focus pixels more clear, you see the few millimeters of the out of focus edges of things become bigger... and you will see that things are out of focus sooner than before you took those steps.
If you take a few steps back, the opposite happens, more appears in focus.
So, but taking steps, you change the depth of field!

Same thing happens when you zoom in and out of a photo. Depth of field is just a perception... So, when you crop a photo (or use a sensor that is smaller) the depth of field WILL change. A 50mm lens on a 1.5x crop Pentax will have a more shallow depth of field than a 50mm lens on a 35mm film Pentax if you have the exact same distance to your subject, because the cropping has a zoom in effect. If you then crop the 35mm photo, the depth of field will be the same again!
It may well be possible that the 75mm lens on a 35mm film camera and the 50mm on the 1.5x crop factor camera give the exact same depth of field perception.