Crop Factors

All the complication aside if your shooting at 85mm on a crop and thats what you prefer then isn't the simple answer just buy a 85mm prime! :thinking:
 
Danny8oy said:
All the complication aside if your shooting at 85mm on a crop and thats what you prefer then isn't the simple answer just buy a 85mm prime! :thinking:
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How will he know that if hasnt got an 85mm yet and is thinking of getting one?

I think Nod's advice on post #4 or #5 is about right. Go through your favourite photos so far, if some of them are at the "reach" or focal length you like and you just want a sharper lens with the same "reach", go for a prime of that focal length. If you think that none of the photos are getting you "close enough", try a longer length prime than your kit lens's longest.
 
FlyingShrapnel said:
Hope you dont mind, im just going to hop on the back of this thread instead of starting a new one.

My question is: A crop factor "gives extra reach" because it is already cropped in camera compared to FF so the subject appears to fill the frame more, thus giving the illusion of "extra reach" right? But surely the compression/"subject isolation" (not bokeh, but how the subject of focus appears compared to the background) effect of the lens stays the same, so 50mm on a crop body would look exactly the same as 50mm on a FF except cropped in towards the centre, instead of an image shot on FF at 80mm even though 50mm x 1.6 = 80mm?
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The problem is that you would never take those two photos in the real world.

The compression is a function of perspective, which is linked purely to the distance between the camera and the subject. If you wanted to take a head and shoulders portrait using an 80mm lens on both a FF and crop, you'd have to stand further back with the crop altering the perspective (and hence compression). Alternatively you could put a 50mm lens on the crop body, which would let you stand in the same spot as the FF and get the same perspective
 
someone did take those two shots ;) http://www.thephotoforum.com/forum/...rence-between-full-frame-cropped-sensors.html

the situation you described is the 3rd pair of images and it confirms you are right, it depends on where you stand. so, to keep the subject the same size, the focal length is a function of how far away you are from the subject. on crop bodies, because of the crop factor, you dont need as long a lens to get the same subject size, so there is such a thing as crop factor :gag:

im so confused... :bang::bang:
 
Crop factor alone ONLY affects the FOV. Everything else that happens is due to the lens, the working distance, or the sensor's pixel density (MP's/pitch/size).

Perspective is due to working distance.
DOF is due to working distance (and lens aperture)
Working distance is due to the lens magnification (FOV)
Magnification is due to the lens' FL (and actual magnification is "better" than crop factor)
Detail is due to magnification and MP's (size)
Print size is due to MP's
Diffraction is due to MP's (size/pitch)

Keep things separate as they should be and it's easy to understand. Start trying to consider all of the possible different combinations that can occur simply by changing between two bodies/sensors and it can get very confusing.
 
sk66 said:
Keep things separate as they should be and it's easy to understand. Start trying to consider all of the possible different combinations that can occur simply by changing between two bodies/sensors and it can get very confusing.
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Which is why it's even more daft to consider the intricacies of what crop factor means when you only have a crop camera.

Unless you own both crop and ff or are planning on buying ff and need to think through your lens options, then it's purely academic, and about as relevant to actual photography as what type of trousers you're wearing.
 
Phil V said:
Which is why it's even more daft to consider the intricacies of what crop factor means when you only have a crop camera.

Unless you own both crop and ff or are planning on buying ff and need to think through your lens options, then it's purely academic, and about as relevant to actual photography as what type of trousers you're wearing.
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The intricacies maybe, but everyone needs to understand the basics, even if they only use one camera. That's exactly what was behind the OP, who was uncertain about what focal length lens to get because of the crop factor.

LOL at how this thread has gone though. Actually, the OP had got it right in post #1, and that was confirmed in post #2 :lol:
 
I'm with Phil. Crop factors aren't any part of the "basics" and have no business being discussed. They are an extra level of unnecessary complexity.

The basic is that (to a good approximation) for any lens image size is directly proportional to focal length. If you want an image twice the size, double the focal length. That shouldn't be beyond anyone.

The other concept is even simpler. When you take an image and cut it in half, you don't get a smaller version of the photo you started from. You get a half of it. And that's all that happens when you change the size of the sensor or the size of the film. You get less in if you reduce the size, more if you increase.

What you need to know in addition as a basic is how much you get in on your camera with a given focal length. Then you estimate how much more or less you want, and can change the focal length accordingly.
 
I think its important to understand what your cameras crop factor means but other than that it doesnt matter. Just know if you want wide angle lens with a dx camera you need to go wider than you actually want I.e 12mm for a true 18mm. End of
 
Sorry but no. All you need to know is what focal length is the "standard" for your body, usually about the same as the diagonal measurement of the recording medium (sensor or film frame). This is about 50mm on FF and about 30 on a crop body (?25mm on 4/3?). Anything shorter is a wide angle and anything longer is a telephoto. Unless you're swapping from FF to crop and back, all you need to know is that your 18-55 is a standard zoom, your 10-20 is a wide angle zoom and your 55-200 is a telephoto zoom. If you have a different lens line up, you're probably comfortable with your familiarity with focal lengths! ;)
 
1) Focal length doesn't change, eg 50mm is always 50mm.

2) It is the field of view that changes with different formats, because the sensor is smaller, ie it's been 'cropped' and records less of the scene.

3) To compare different formats to full-frame, divide focal length by the crop factor to get equivalent field of view.

- 50mm on full-frame has same field of view as:
- 33mm with 1.5x crop factor (Nikon/Pentax/Sony)
- 31mm with 1.6x crop factor (Canon)
- 25mm with 2x crop factor (Micro 4/3rds)
 
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