View Full Version : Question regarding sample test on CPP website.

02-10-2012, 05:25 PM
I'm taking my certification test tomorrow. *I took the sample test on the cpp website. *I got them all correct except one. *I need help. *I'm not sure I'm understanding the question. *Since it was a practice test I'm assuming there's a good chance the concept will be addressed on the test.

Here's the question:

If focusing on the same subject at the same distance, what will happen to the image size if a 300 mm telephoto lens is used on a 35 mm camera compared to a 300 mm lens used on a medium format camera.

1. * The image size with the 35 mm will be slightly larger
2. *The images size with the medium format will be larger
3. *There will be no difference in the image size
4. *The image size with the 35 mm will be smaller

I don't know a whole lot about medium format cameras but i assumed the question was addressing the same problem found with different sensor sizes. *I know that on my 40D with a 3/4 sensor the image at the same focal length is larger (more magnified) than on my 5d with a full sensor. *

I found a website regarding medium format cameras that state:

What you'll see is that you need to use a factor of about .6X to convert for 6X6cm and 645cm, and .5X for 6X7cm. In other words, a 100mm lens on a 6X7cm camera has a field of view similar to that of a 50mm lens on a 35mm camera. An 80mm lens is similarly "normal" on a 6X6cm camera.

Therefore I think the answer should be "None of the above." as the image on the 35 would be up to twice as large as the medium format. To use the example above:

"a 300mm lens on a 6X7cm camera has a field of view similar to that of a 150mm lens on a 35mm camera."

The correct answer according to the cpp site is "C" -No difference.

I must be mis-reading the question. *Can someone please help?

02-10-2012, 05:38 PM
Pamela, "C" is the correct answer. A 300 mm telephone produces the same images size at a given distance. Yes, sensor size and film size could crop or change what you see on print, but the image "in the camera" from a 300 mm lens on each camera will be the same. Think of in terms of the lens only. The focal length of 300 mm, the distance from the film or sensor to the focal point in the lens is the same.

02-10-2012, 06:19 PM
The smaller sensor doesn't have a "zoom" factor. It has a "crop" factor. The resulting image is the same size on both but cropped more on the 35mm.

02-10-2012, 06:45 PM
Ok. Got it. Thanks for the clarification.

02-10-2012, 09:29 PM
You're not the first person to struggle with this concept. The easiest way to think of it is from the lens' point of view. A lens has a specific focal length to focus an image. Let's say you have a 300 mm lens - you mount it on a simple focusing device, an 8x10 view camera which is just a lens, a focusing screen, and a bellows in between. If you were looking through the back of an 8x10 view camera fitted with a 300 mm lens, the lens focuses the image on the ground glass and you get a certain size image and that image size is dependent upon the focal length of the lens. Now you bring out a bunch of film holders each holding a different format film or digital sensor - one holding 35 mm film, one holding medium format roll film, one holding 8x10 film, one holding 4x5 film, and one holding a full frame digital sensor and one holding a crop sized digital sensor. You put the image on film or sensor by placing the holder at the focusing plane on the back of the camera. There is nothing you can do to change the size of the image created by that lens - all you can do is to capture a larger or smaller portion of that image. If you put the 8x10 film holder at the film plane, you capture the entire image. When you place the 4x5 film holder at the film plane, you capture less, so it looks like you've zoomed in some. The medium format film will cover an even smaller portion of the film plane so it looks like you've zoomed in even more, but you really haven't. When you place the crop sensor digital back at the film plane, you capture only a very small portion of the image - so it looks like you've really zoomed in on the image with that sensor, but you haven't actually zoomed in any, you've just captured or cropped out a small portion of the image produced by that lens. The same thing happens in your camera - with longer lenses, you're just seeing a small central spot of the image that the lens has produced cropped for your camera's sensor.