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dslr focal lengths?
Could someone explain what the formula is for converting the focal length for the lens of a dslr into an slr lens (i.e. what does 18mm - 50 mm in a digital slr really mean in terms of an slr)? Furthermore, why is there a difference at all? Thanks.Happy Snappin'James..

Comments (11)

The actual focual length of the lens doesn't change. But the field of view (how much of the scene is in the picture) does..

The reason is that most DSLR sensors are smaller than the 35mm negative/slide film frames around which people developed their expectations. So the sensors essentially are taking only the middle of the picture, then blowing up that part, and presenting it to you as if that _was_ the entire picture..

Common crop factors for affordable cameras:.

Nikon (DX) format; Pentax; Konica-Minolta/Sony 1.5xCanon 1.6x.

Olympus (and other "Four Thirds") manufacturers 2x (and in a different aspect ratio).

So, e.g. a 50mm lens would "act like" a (50 x 1.6) = 80mm lens on a Canon 400D. But if you wanted a lens that "acted like" a 50mm does on film, you'd need a (50 / 1.6) = 31.25mm lens (and would probably have to settle for a 28mm, 30mm, or 35mm nobody makes one that's exactly 31.25mm)...

Comment #1

The conversion factor depends on the sensor size so is variable for different camera... just to add to the confusion..

Imagine a lens of a focal length of (say) 50mm generating a large circular image. Now draw in the centre of that circle rectangles of different sizes, corresponding to different sensor sizes (or, if you prefer, different film negative sizes). A 35mm negative (an area of 24 x 36 mm) covers about as much of the field of view as you would see with your eye, which is why 50mm was 'standard' for 35mm format cameras..

Now draw a smaller rectange of 15 x 22 mm. Only a smaller part of the image is captured. When this is blown up to the same size as the previous example, the effect is like that of a mild telephoto lens, i.e. concentrating only on a smaller part of the image. The conversion factor is about 1.5, giving an 'equivalent' focal length of 75mm..

If you have a tiny sensor (like on a compact), you would only capture a very small part of the image, just like a powerful telephoto. So the smaller the sensor, the more powerful is the telephoto effect of your lens even though it's focal length doesn't change..

Now draw a large rectange, say 6 x 4.5 cm (medium format film). You will have captured a much larger part of the image, equivalent to the field of view of a wider angle lens, and indeed 50mm would count as a wide angle lens on a medium format camera..

For most common DSLRs with a 15x22mm sensor, the conversion factor from 35mm film days is about 1.5, so a 'standard' 18-55mm lens is equivalent to about 27-82mm on a 35mm camera, which used to be a standard zoom range..

On the Olympus DSLR series, which have a alightly smaller sensor, the conversion factor is 2. This is why their standard kit zoom is 14-42 mm, equivalent to 28-84mm for a 35mm camera, and equivalent to 18-55 on most other DLSRs..

On the top-end Canon EOS-1D, which has a 'full-frame' sensor the same size as a 35mm negative, the conversion factor is therefore 1 - no change from 35mm film..

I hope that helps....best wishesMike..

Comment #2

Thank you both so much. It finally makes sense.Happy Snappin'James..

Comment #3

Delighted..

One more point that may be relevant to you. Lenses designed for 35mm film cameras produce an image circle that more than covers the area of a 24 x 36 negative (obviously), so they can be used on modern DSLRs which generally have smaller sensors, giving an 'effective' focal length of around 1.5x higher..

The converse is not true however. Many lenses now are made that produce a reduced image circle, big enough to cover only a 15 x 22mm sensor (not this is not a focal length issue as the magnificaition of the image is the same; but the lens elements have a smaller diameter). This makes the lenses smaller and lighter which is great, but you cannot use them on 35mm film cameras, or even a DSLR with a full-frame sensor. The image circle will not reach the edges of the larger frame and 'vignetting' will occur, cutting the picture off towards the edge of the frame as though the picture were taken through a porthole. So you could use lenses from a 35mm film camera on a DSLR, but not usually vice versa unless the lenses are specifically designed to cover a 'full frame' sensor..

Best wishesMike..

Comment #4

Tom_N wrote:.

The actual focual length of the lens doesn't change. But the fieldof view (how much of the scene is in the picture) does..

The reason is that most DSLR sensors are smaller than the 35mmnegative/slide film frames around which people developed theirexpectations. So the sensors essentially are taking only the middleof the picture, then blowing up that part, and presenting it to youas if that _was_ the entire picture..

Common crop factors for affordable cameras:.

Nikon (DX) format; Pentax; Konica-Minolta/Sony 1.5xCanon 1.6xOlympus (and other "Four Thirds") manufacturers 2x (and in adifferent aspect ratio).

So, e.g. a 50mm lens would "act like" a (50 x 1.6) = 80mm lens on aCanon 400D. But if you wanted a lens that "acted like" a 50mm doeson film, you'd need a (50 / 1.6) = 31.25mm lens (and would probablyhave to settle for a 28mm, 30mm, or 35mm nobody makes one that'sexactly 31.25mm)..

With the explanation above it appears that ONLY expensive models like eos-1D, Nikon D3 got full frame sensor with conversion factor of 1, i.e. no change. (wow, that is going to cost a bomb for a newbie dslr owner).

Is there any connectors/adaptors that can make the conversion factor back from 1.5x back to 1 so that I can enjoy an 18mm lens that really start from 18mm (instead of 18 x 1.5 = 27mm equivalent). Thanks in advance..

Comment #5

Commi wrote:.

Is there any connectors/adaptors that can make the conversion factorback from 1.5x back to 1 so that I can enjoy an 18mm lens that reallystart from 18mm (instead of 18 x 1.5 = 27mm equivalent)..

Sure, you could screw a wide adapter on to the front of the lens. Not usually great of image quality, though..

Seriously, just buy a new wide (or ultra-wide) and be done with it. Your 18mm is no longer ultra wide, it is wide. Your 50mm is no longer normal, it is short tele. Your 200mm is not tele, it is long tele. Etc..

Or buy a camera with a 35mm sensor (assuming your existing lenses work with one)..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #6

I found this discussion helpful:http://www.bobatkins.com/photography/technical/field_of_view.html.

Also, contemplating the original question of this thread, I've come to the conclusion that there must be a developed sense on the part of the 35mm film users (or other size film or sensor users) about angle of view (i.e., picture angle or field of view); that is, a sense of whether the object of interest is within our without the angle of view, for example a 39 degree 40 minute angle. I wonder if this 'extra' sense exists? And I wonder if there is ar tool or instrument which yields the required info on angle of view bounds for any shot?.

Note: I use the AF Micro-Nikkor 60mm f/2.8D lens with the Nikon D80. I know when the object is out of view when there is a failure to focus given the desired distance from camera to object..

Http://www.umefotographie.com..

Comment #7

Gking wrote:.

And I wonder if there is ar tool or instrumentwhich yields the required info on angle of view bounds for any shot?.

Yes, and you've probably got one of these instruments attached to you:.

Http://www.fredparker.com/vislize.htm.

You don't have to go to the extreme knowing all of them, just knowing 50mm, 100mm, and 200mm (all "35mm equivalent") will be close enough to get you in the correct ballpark..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #8

The bounds are determined by the length of an arc which is subtended by the angle of view. Illustration: given an angle of view of 45 degrees (radians= 0.79) and a distance of subject (radius of a circle) =100 feet; then using the formulae :.

Angle in radians = arc/radius,the length of the arc can be solved for and equals=79 feet..

The mental acuity to project this arc with a generous standard deviation of say plus or minus 20 feet would be impressive..

Http://www.umefotographie.com..

Comment #9

Mike703 wrote:.

The converse is not true however. Many lenses now are made thatproduce a reduced image circle ... This makes the lenses smaller andlighter which is great, but you cannot use them on 35mm filmcameras, or even a DSLR with a full-frame sensor..

I believe Nikon's recently-announced full-frame digital camera has changed the rules. It can operate in "full frame" mode with full-frame lenses and in "crop frame" mode (my term for it, not Nikon's) with reduced-image-circle lenses..

In the "crop frame" mode, it uses just the middle part of the sensor (the part that corresponds to the area the reduced-image-circle lenses are designed for). Presumably you have to compose using just the middle part of the viewfinder...

Comment #10

Commi wrote:.

Is there any connectors/adaptors that can make the conversion factorback from 1.5x back to 1 so that I can enjoy an 18mm lens that reallystart from 18mm (instead of 18 x 1.5 = 27mm equivalent). Thanks inadvance.

Time for you to go shopping for an (18mm / 1.5) = 12mm lens. Probably in the form of a reduced-image-circle zoom, if you want one that doesn't cost an arm & a leg .....

Comment #11

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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