SonyA580 Loc: FL in the winter & MN in the summer

I was just reading the thread on DOF and downloaded a DOF calculator. As I plugged in the numbers I came up with a question I can not answer: when using a FF lens (old Minolta) on my Sony APS-C body, would the DOF be calculated on the FF lens dimension. i.e. 100mm or, the DX equivalent lens dimension, 150mm, all other things remaining equal?

speters Loc: Grangeville/Idaho

It really has nothing to do with FF lens or APS-C lens. The main factor is the sensor and what lens-settings you're using, focal length and aperture!

SonyA580 Loc: FL in the winter & MN in the summer

What I'm asking is, when using the DOF calculator, which dimension should I plug in for "Focal Length", the FF or the crop factor? Thanks for the reply.

GoofyNewfie Loc: Kansas City

The actual focal length.
If what you have is a 100mm lens, that's what you plug in, not the FF equivalent.

SharpShooter Loc: NorCal

Sony, it can't possibly give you that choice, can it?
The lens is, what it is. 50mm is always 50mm. Don't confuse yourself. ;-)
SS

SonyA580 Loc: FL in the winter & MN in the summer

The Nikonian DOF App for I-Phone asks for "Focal Length", "Aperature" and "Distance". I am assuming the "Focal length" is the focal length of the lens. Am I wrong?

Wall-E Loc: Phoenix, AZ

Yes. NOT the 'apparent' length, due to crop factor.
So if it's a 50mm prime, then you put in 50mm.
That's accounted for when you input either your camera or the sensor size.

SharpShooter Loc: NorCal

Sony, yes, you ARE very correct.
By the way, welcome to the Hog, where honey and a hornets nest are almost the same thing. ;-)
Not too long ago we had almost the same exact question. It's a very fine line of semantics. In that tread, which went on for quit a while, the ambulances were quite busy, hauling the injured, victims of falling un-dotted I's and uncrossed T's. It all started innocently enough!
But Sony, it's an excellent question that can get VERY complicated, and when it does, your "circle of concussion";-), just gets bigger.
Yes, your querry has been answered.
Good luck
SS

joer Loc: Colorado/Illinois

You use the FL marked on the lens. The camera choice takes care of the crop factor. Don't worry about equivalencies.

selmslie Loc: Fernandina Beach, FL, USA

Correct. A 50mm lens is 50mm regardless of sensor size.

But to muddy the water a little, look at the DOF calculator at http://www.cambridgeincolour.com/tutorials/dof-calculator.htm and click on show advanced. Read the accompanying explanation to get a full understanding of the topic.

DOF is not just a simple calculation based on distance, aperture and sensor/film size. It depends also on print dimension, viewing distance and eyesight.

f8lee Loc: New Mexico

Okay, I'm going to enter this fracas based on the readings from the iPhone app I use - DOF Master.

Using a 200MM lens set at f2.8 and focused at 40 Meters, here is what it calculates for DX and FX formats:

FX - near limit 36.9Meters, far limit 43.7 Meters, total DOF 6.8 Meters - hyper focal distance - 471.6 Meters
DX 37.9 42.4 4.52 707.3

In other words, the narrower angle of view afforded by the DX format (i.e. - the "crop factor" that makes it as if it were a 300MM lens on an FX body) seems to follow the rule of thumb that wider angle lenses have greater DOF than longer focal lengths. My guess is this has to do with the "Circle of confusion" and the apparent distance the subject is from the camera (remember, she's standing 40 meters away). But them's the numbers from the chart.

I believe the explanation is that the smaller sensors require a smaller CoC since the captured image will have to be enlarged more to get the same end result as an image taken with a larger sensor.

amehta Loc: Boston

Yes, your explanation is correct. These are all calculated from an old definition of CoC which considered looking at an 8x10 print from 1 foot away, and they decided that we could perceive differences of 0.01 inch. Since the sensors are different sizes, the corresponding CoC are different sizes, and that affects the DoF calculation.

lovitlots Loc: Tottenham, Ontario, Canada

When you use a FF lens on a crop factor sensor, the only thing that changes is how much of the image cone gets recorded. Basically you've cropped the image produced by the ff lens. If the lens has a focal length of 50mm for example, it will still be 50mm on the crop sensor. But because the aps-c sensor only sees a smaller portion of the image produced by the 50mm lens it appears that you're using a 75mm lens, if your crop factor is 1.5. That's one of the beauties of the crop sensor, you can take take a ff lens, automatically increase it's magnification by 1.5 but still maintain the dof of the ff lens. The crop factor is different for some camera brands but in your case it is 1.5 .

amehta Loc: Boston

It sounds like you just said that DoF does not depend on the sensor size, but it does.

lovitlots Loc: Tottenham, Ontario, Canada

No it doesn't. It's solely dependent on the focal length of the lens and the aperture used. Think about it for a moment. FF camera and 100mm lens and f5.6. Now CS camera, same 100mm lens set a f5.6. The only thing affecting Dof is the lens's focal length and aperture used. Since they stay fixed in the senario the dof stays the same. The sensor size only determines how much of the image area the camera records. The crop sensor doesn't actually change the focal length of the 100mm lens or the aperture.