depth of field 50d vs rebel xt

is the dof different on the rebel line of cameras compared to 40d/50d??
i find that f5.6 @200mm on my 50d gives a much more blurred background compared to my rebel at the same settings with the same lens
on the other hand i find that with landscape shots with the 50d i have to go to f13 or more to get everything in focus compared with f8 or f9 on my rebel

patrick clarke wrote in post #8420424
is the dof different on the rebel line of cameras compared to 40d/50d??
i find that f5.6 @200mm on my 50d gives a much more blurred background compared to my rebel at the same settings with the same lens
on the other hand i find that with landscape shots with the 50d i have to go to f13 or more to get everything in focus compared with f8 or f9 on my rebel

Both are 1.6x crops, so you should have the same do. Have you tested with the same distance? As far as I know they should perform equally.

Yes, theoretically they should be the same in terms of DOF. Have you tried setting up a tripod and taking a photo with each camera with the exact same settings? That would be interesting to see if your results are different.

Camera should not have an effect on DOF, it is a function of FL and distance to subject. Try a battery or crayon focus test with both bodies using the same lens, same settings to see what your results are.

Are you viewing at 100% onscreen or at same-size prints?
DoF comparisons only make sense when viewing equal size enlargements (e.g. same size prints). 100% on screen is a greater enlargement (22x) for the 50D than it is for the XT (16x) so more will seem blurred in the 50D shot simply because of that factor.

There will be no difference. Your perception is being based on 2 images shot or viewed under different conditions.

patrick clarke wrote in post #8420424
is the dof different on the rebel line of cameras compared to 40d/50d??
i find that f5.6 @200mm on my 50d gives a much more blurred background compared to my rebel at the same settings with the same lens
on the other hand i find that with landscape shots with the 50d i have to go to f13 or more to get everything in focus compared with f8 or f9 on my rebel

DOF is a function of the lens aperture and focal length only since the lens mount to sensor plane distance (or film plane) is the same in all cases. Active sensing area whether film or electronic sensor is irrelevant.

The term "depth of field" as applied to photography is somewhat nebulous since it is somewhat arbitrary with respect to what is acceptably "in-focus". The term makes a bit more sense when looking through a telescope or binoculars since we can use the angular resolution of the human eye as an approximate gauge. However, with photography, too many unquantified factors are also involved. Some of these include printer, ink, and paper characteristics along with size if printing. If viewing on a monitor, thing like JPG compression, post processing image resizing algorithm, monitor dot pitch and other display characteristics affect the results. On top of all this, if we are using vision correction lenses, that also becomes a part of how sharp a displayed image looks.

POP QUIZ: Close your books and no peeking at other people's answers. Do your own work. Answer the following with either "yes" or "no" (HINT: the correct answer is "NO"). This is a timed exam -- you have one minute. Allow an extra minute for the extra credit questions.

Suppose you had a 35 mm film camera which has an image area of 864 mm² (36 mm X 24 mm) -- after developing the film negative suppose you used scissors to trim the size to 22.5 mm by 15 mm (which is ~ 338 mm² or 39 % of the larger film area). Would doing so cause the depth of field to change?

Extra Credit: Explain why an electronic sensor that has an active area which is 39% of the standard image size for 35 mm film is referred to as a 1.6 crop.

Part 2 of extra credit: Does 1.6 sound better than 0.39? Write a 25 page thesis explaining your answer.

Bill Boehme wrote in post #8420699

POP QUIZ: Close your books and no peeking at other people's answers. Do your own work. Answer the following with either "yes" or "no" (HINT: the correct answer is "NO"). This is a timed exam -- you have one minute. Allow an extra minute for the extra credit questions.

Suppose you had a 35 mm film camera which has an image area of 864 mm² (36 mm X 24 mm) -- after developing the film negative suppose you used scissors to trim the size to 22.5 mm by 15 mm (which is ~ 338 mm² or 39 % of the larger film area). Would doing so cause the depth of field to change?

Let's say you took two shots with identical settings and cut one up but not the other. If you looked at the two pieces of film from the same distance, the DoF will be the same. If you put them in the same projector and kept the projector in the same place and viewed the images on the wall, the DoF would be the same.
If you make a print in the enlarger at the same settings for each piece of film and view the resulting prints from the same distance, the DoF will be the same (although the cut-up piece will yield a smaller print).

If you make a print from each piece of film and the prints are the same size and viewed from the same distance, the DoF will not be the same.

TeamSpeed wrote in post #8420554
Camera should not have an effect on DOF, it is a function of FL and distance to subject.

Wrong, wrong, wrong. ALL lenses have the same DOF. I'll repeat, ALL lenses have the same DOF. DOF comes from (1) f-stop (2) relative size of image. I've done the tests myself. My 20mm lens has the same DOF as my 200mm does, at the same aperature, when image size is held constant in the viewfinder. Cameras CAN have an effect on DOF also, if the format size is different. A 90mm lens on my 4x5 large format camera has somewhat less DOF than a 90mm lens will have on a 35mm camera, maybe one f-stop less. There should be absolutely NO difference in DOF between the two cameras mentioned though, since they are the same format size.

These forums can be used to educate people and finally put those old myths to rest:

http://www.luminous-landscape.com/tutorial​s/dof2.shtml

Kent in SD

AJSJones wrote in post #8420771
Let's say you took two shots with identical settings and cut one up but not the other. If you looked at the two pieces of film from the same distance, the DoF will be the same. If you put them in the same projector and kept the projector in the same place and viewed the images on the wall, the DoF would be the same.
If you make a print in the enlarger at the same settings for each piece of film and view the resulting prints from the same distance, the DoF will be the same (although the cut-up piece will yield a smaller print).

If you make a print from each piece of film and the prints are the same size and viewed from the same distance, the DoF will not be the same.

Let's add to your example that the subject is an inclined ruler (your choice of inches or centimeters). Cropping down the negative and then using the enlarger to fit the image onto the same size paper as the uncropped negative is just another way of saying that we are using the enlarger to apply more magnification. What will the results look liked when the printed images are compared? The ruler in one print will be larger than it is in the other, but will that make the sharply focused area appear greater in one than the other since our brain has a context to judge the images?

Physiological characteristics of vision are important factors that need to be considered -- for example, our brain automatically performs white balance correction under different lighting conditions because we have learned what is supposed to be white. Another physiological characteristic is that absolute size is not as significant as relative size when we can identify something in the subject that gives us a context for identifying actual subject size within reasonable limits, of course. Anything would look sharp at postage stamp size or blurry at billboard size if the viewing distance were the same.

Two23 wrote in post #8421007
Wrong, wrong, wrong. ALL lenses have the same DOF. I'll repeat, ALL lenses have the same DOF. DOF comes from (1) f-stop (2) relative size of image. I've done the tests myself. My 20mm lens has the same DOF as my 200mm does, at the same aperature, when image size is held constant in the viewfinder. Cameras CAN have an effect on DOF also, if the format size is different. A 90mm lens on my 4x5 large format camera has somewhat less DOF than a 90mm lens will have on a 35mm camera, maybe one f-stop less. There should be absolutely NO difference in DOF between the two cameras mentioned though, since they are the same format size.

These forums can be used to educate people and finally put those old myths to rest:

http://www.luminous-landscape.com/tutorial​s/dof2.shtml

Kent in SD

Okay, I will correct myself a little here however there is a "but" that follows. DOF is dependent on aperture and distance to the subject, like Bill states above. However aperture is dependent on FL (f-stop = focal length / diameter of lens opening).

When you say it comes from the relative size of the object, then you are talking about the distance to the object or your focal length. How do you make the subject the same size on the frame with two different lenses? You either change your distance to the object physically or you use a zoom lens and change the FL...

patrick clarke wrote in post #8420424
is the dof different on the rebel line of cameras compared to 40d/50d??
i find that f5.6 @200mm on my 50d gives a much more blurred background compared to my rebel at the same settings with the same lens
on the other hand i find that with landscape shots with the 50d i have to go to f13 or more to get everything in focus compared with f8 or f9 on my rebel

How are you viewing the images to come to your conclusions? If you are looking at the images on a computer display at 100% resolution (where you need to scroll around the image to see it all), you may be confused by different resolution between the two cameras.

If you display an 8 megapixel image at 100% and compare it to a 15 megapixel image at 100%, you are effectively displaying the 15 megapixel image at a MUCH larger physical size than the 8 megapixel image. That would make any blur in the background seem greater with the 15 megapixel image, as you are displaying everything in the image larger.

Try looking at comparison images from the two cameras at the same size or print the images to the same size. If you've used the same camera-subject distance, the same focal length, and the same aperture (f-stop) setting, the depth of field will be the same and you should see that in identical-sized images.

TeamSpeed wrote in post #8421794
When you say it comes from the relative size of the object, then you are talking about the distance to the object or your focal length. How do you make the subject the same size on the frame with two different lenses? You either change your distance to the object physically or you use a zoom lens and change the FL...

You can also do it by cropping the image. Saying that DOF is a function of focal length misleads people and can cause problems in the real world. I'll give an example. I was once trying to take a photo of three deer skulls lying in the woods. I was using a Bronica ETRSi with lenses 40mm, 75mm, 150mm, 250mm. I first tried the 150mm, but could only get two of the three skulls in focus (DOF.) I then thought the 75mm lens would have more DOF so I switched to it. I moved in closer to preserve framing, and found DOF was the same. I put on the 40mm--same thing. I then put on my 250mm and backed way off, hoping "telephoto compression" would get everything in focus. Result was the same. With the framing held constant, all four lenses had exactly the same DOF. So, I used the 40mm lens and backed off, took a shot. I made a print and cropped it. DOF was the same as before with all the other lenses.

My point is that by believing the myth, I lost about an hours' time and did not get the shot. I now have tilt/shift lenses and can get those shots. By eliminating the myths such as wide angles have more DOF, I become a more knowledgeable photographer and get the shots I want.

Kent in SD

Bill Boehme wrote in post #8421045
Let's add to your example that the subject is an inclined ruler (your choice of inches or centimeters). Cropping down the negative and then using the enlarger to fit the image onto the same size paper as the uncropped negative is just another way of saying that we are using the enlarger to apply more magnification. What will the results look liked when the printed images are compared? The ruler in one print will be larger than it is in the other, but will that make the sharply focused area appear greater in one than the other since our brain has a context to judge the images?

Physiological characteristics of vision are important factors that need to be considered -- for example, our brain automatically performs white balance correction under different lighting conditions because we have learned what is supposed to be white. Another physiological characteristic is that absolute size is not as significant as relative size when we can identify something in the subject that gives us a context for identifying actual subject size within reasonable limits, of course. Anything would look sharp at postage stamp size or blurry at billboard size if the viewing distance were the same.

The physiological characteristics of vision are the very basis for DoF - objects (such as markings on a ruler) that appear as sharp as the sharpest areas in the image (as printed to a certain size and viewed from a certain distance) are defined as being within the DoF. (The sharpest areas in any given image are the context for the brain that you refer to). As soon as the marking seems to be a little softer than the one next to it, you have set the edge of the DoF. At that point, the eye has just been able to distinguish a pinpoint from a slightly blurred pinpoint on the print.

Let's keep going with the example and say I have acuity and can see 254 ppi detail in the print from my viewing distance (without my glasses I don't stand a chance ). That means those pinpoints I can see are 0.1mm wide on the print. If something is less than 0.1mm wide (say, 0.08 or 0.07) I still perceive it as a pinpoint (i.e. sharp), but if it gets up to 0.12 or 0.15 I can see it's bigger (and therefore not as sharp). So 0.1mm is my threshold for being able to detect an OOF pinpoint in the image.

Let's imagine a lens that's better than the film and is not limiting this dicsussion. Also that the markings on the ruler are so finely engraved that they can produce an image in the print that is 0.05mm for something that is at the focal plane - the only place that is actually in true focus. Markings on either side of that will be perceived as pinpoints (i.e. deemed to be in focus) until we get far enough away that they produce an image that is > 0.1mm and we say Aha - that one's out of focus and note the distance on the scale. One for the near point and one for the far point.

Now, we make a 1.6x blow-up by printing the cut-up film to the same size as the uncut film. The sharpest marking is now 1.6 x 0.05 mm in the print , i.e. 0.08mm. I still see it as a pinpoint, so this looks just as sharp as it did in the original print because it's below 0.1 threshold. The marking that produces an image on the print that used to be 0.1mm will now be 0.16mm and is definitely seen as different. What used to give 0.063 will now give 0.1 and be a pinpoint but it's a different point on the scale from the threshold before. We now see, that as a result of blowing the image up more, we have changed the range of markings that appear, to a given eye at a given distance, as pinpoints and the range on the ruler where that happens will be smaller. Degree of enlargement has just changed DoF. It's not an intrinsic property of the film, but how the image is made from it and then how it's viewed.

Did I pass?? (I know I prolly used more than the allotted time )

I feel like I've been circled by confusion!