cropping photos... reworded for your enjoyment ;)

[jann1033]

when you crop a photo, is there any kind of set formula as to what the mm would appear to be in the finished photo? ie if i use a 100mm lens and crop out 1/2 the photo would the results appear to be 150mm? ( know it wouldn't have all the details etc, i mean just the way it looks). i wonder about this as i would like to see how long a lens i need...most of the photos i take with my 135mm and crop, i probably crop to about 2/3 the original size to get to where i want it to be so i wondered it that would mean a 200mm lens would be ok for me usually

 

[jann1033]

guessing this in the unanswerable question but bumping just in case....i'll try rewording...does a crop of 100 % = that much more appearance of more focal length ie 100mm lens' photo at 100% crop looks like 200mm lens' photo? is there any correlation at all or am i just not able to use how much i crop something to figure out how much zoom i need?

 

[boBQuincy]

You have it right, if you crop the image to 50% (linear) it is the same as using a lens twice as long (except for fewer pixels). In hsi book "The Camera" Ansel Adams did just this, taking photos from the same spot with different lenses to show that only the magnification changed.

If you are often cropping more than just a little to "clean up" the image then you probably would like a longer lens.

Interestingly, one theory was that as pixel count increased we would no longer need long lenses, with our 200 MP cameras we could just crop for the central 10% of the image and still have high quality images. I'm still waiting...

 

[MarkBarbieri]

If you take a picture with a 100mm lens on a Digital Rebel (1.6x crop factor) of a subject 100 feet away, you will get a picture that shows a 22' 6" wide and 15' 0" tall section of your subject. If you switch to a 200mm lens, you'll get a picture that is 11' 3" wide and 7' 6" tall. I learned that by plugging in those values on the Dimensional Field of View Calculator on this page.

Doubling the focal length cuts the horizontal coverage in half and the vertical coverage in half. The converse would also hold true; taking a picture shot at 100mm and cutting down to the middle part that is half as wide and half as tall would give you the same scene that a 200mm lens would have seen. Of course, you'll only have 1/4 of the pixels, but it will cover the same area.

One thing that doesn't change is perspective. A lot of people are convinced that wide angle lenses have a different perspective than telephoto lenses. They don't. Perspective is strictly a function of the relative position of the camera and the subject. If you shoot the same scene with a wide angle lens and a telephoto lens from the same place, the telephoto shot has the same perspective but covers a smaller portion of the subject.

If you shoot a subject at 100 feet with a 135mm lens on your Rebel, you'll get a 16' 8" wide by 11' 1.3" tall portion of your subject. You say that you would typically crop to 2/3 of that. I'll assume that means that you'll cut 1/3 off the vertical and 1/3 off of the horizontal. That leaves you with a subject that is about 11' 1" wide by 7' 5" tall. That's roughly the equivalent of a 200mm lens. We could easily have done the math the other way and just said that 135 divded by 2/3 = 202.5.

I'm not really sure that I understood your quesiton, so I just praddled around the subject a bit in the hopes that I'd stumble over answer. It sure beats watching political speeches.

To be safe, I recommend that you by a 70-200mm f/2.8 IS, a 70-300mm IS, and 70-300 DO IS (for traveling light), and a 100-400 IS. See which one works best for you and send me the rest. Being a charitable sort, I'll find a good home for them.

 

[jann1033]

thanks bob and mark my question is answered ! and mark i will send you my "reject" lenses when you trade me camera bodies