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11-24-2010, 09:23 PM   #1
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A matter of math

I'm a tad confused about how to calculate my focal length while using a Vivitar 2x teleconverter, a 55mm lens and a K110d body.

My question is:

Do I multiply the lens focal length (A) by the teleconverter (B) then multiply that product by the 1.5 multiplier on the body (C)?

so it would be A=55, B=2, C=1.5.

So my 55mm lens turns into what? 55x2=110...110x1.5=165?

That means my 135mm turns into a 405mm?!

11-24-2010, 09:32 PM   #2
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Exactly. Pretty cool, eh? :-)

Just don't be expecting tack-sharp images as if you had an actual 400mm lens.
Teleconverters have their trade-offs, one of which is sharpness.
Another is a loss of light, I think its a four stop loss for s 2x converter. (its too late for me to do math reliably, someone correct me if I'm wrong)
11-24-2010, 09:33 PM   #3
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QuoteOriginally posted by wtf_cowboy Quote
I'm a tad confused about how to calculate my focal length while using a Vivitar 2x teleconverter, a 55mm lens and a K110d body.

My question is:

Do I multiply the lens focal length (A) by the teleconverter (B) then multiply that product by the 1.5 multiplier on the body (C)?

so it would be A=55, B=2, C=1.5.

So my 55mm lens turns into what? 55x2=110...110x1.5=165?

That means my 135mm turns into a 405mm?!
Sensor size has NO effect on the focal length of a lens.

Focal length is 2X 55mm = 110mm, however, the angle of view of that combination on K110d crop-sensor will be the same as 1.5X 2X 55mm = 165mm lens on a full-frame sensor or 35mm film camera.
11-24-2010, 09:40 PM   #4
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Just like you double the focal length you double the stop. f/1.4 = f/2.8, f/16=f/32.

Also as I'm fooling around with it, I'm realizing how it makes the images glow around the edges. Its a nifty ethereal effect without any post processing but could get annoying if not intended. I'm having mixed feelings about this converter. A nice budget zoom for a pinch, but not much more seeing it forces me to tripod my camera.

11-24-2010, 09:49 PM   #5
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Thanks. So its two stops and therefore 1/4 the light. I didn't think I had that quite right.

...and sorry I didn't catch the issue with focal length.
Focal length also affects depth of field.
Depth of field does not strictly change on a crop sensor nor with a teleconverter.
11-24-2010, 10:43 PM   #6
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You only have to worry about crop factor when you switch from one system (being it a SLR or a PS or ...) to another system (e.g. Pentax dSLR). For most people this is probably once in their life.

After that, forget about it.
11-25-2010, 05:54 AM   #7
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Just for the record, multiplication can be permutated. 2x3x4 = 2x4x3 = 3x4x2 etc.
11-25-2010, 06:07 AM   #8
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QuoteOriginally posted by amoringello Quote
Focal length also affects depth of field.
Depth of field does not strictly change on a crop sensor nor with a teleconverter.
Note, the depth of field with a TC and lens will be the same as a lens of the same aperture and focal length as the equivelent of the lens and TC combined.

As for DOF and crop factor, strictly speaking, does not change, between a lens used on full frame and on a cropped sensor, when an image is taken from the exact same spot, and the image is blown up to the same total magnification.

BUT here is where things go wrong. DOF is based upon blowing up/enlarging an image using the entire frame, from the sensor/film, into an 8x10 print. Therefore, just as field of view for a lens changes for the cropped sensor, so does depth of field, because the enlargement ratio from full frame to the 8 x 10 print changes.

If you read into the literature/postings just a little, people talk about circles of confusion (i.e. the diameter of a point where it is still considered a point), at 30 microns for full frame and 20 microns for a cropped sensor. This is simply in response to the final enlargement ratio being higher with a cropped sensor

11-25-2010, 08:39 AM - 1 Like   #9
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QuoteOriginally posted by bdery Quote
Just for the record, multiplication can be permutated. 2x3x4 = 2x4x3 = 3x4x2 etc.
Excellent point. Bravo bdery!
11-26-2010, 12:58 PM   #10
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I'm confused about what you mean by "sensor size" and "cropped sensor."
11-26-2010, 04:10 PM   #11
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QuoteOriginally posted by wtf_cowboy Quote
I'm confused about what you mean by "sensor size" and "cropped sensor."
sensor size is exatly what it says.

we are discussing the size of the CCD or CMOS sensor in the camera.

Pentax uses ASP-C sensors which are nominally 16mm high by 24mm wide. When we discuss the Digital camera and compare the field of view of the lens, you have to consider that with the 24mm high by 36mm wide negative area on film, the DSLR is "cropped" by the ratio 2:3 therefore when people start discussing the "equivelent focal length on digital, they divide the foval length by 2/3 or multiply it by 1.5 multiplier which every one takls about
11-27-2010, 11:55 AM   #12
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I see, I have much to learn. Math was never my strong suit anyway...
11-30-2010, 03:01 PM   #13
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QuoteOriginally posted by wtf_cowboy Quote
I see, I have much to learn. Math was never my strong suit anyway...
Here is the simple, basic equation: SPEED = MONEY

The faster you want a lens to be, the more you pay for it. And the less you pay, the slower it will be, usually. A telextender / teleconverter / TC is a cheap way to get a longer lens, but the cost is speed.

Keep in mind that all telephoto lenses have a TC built into them. This "telephoto group" makes the lens physically shorter than its actual focal length. For instance, 100mm is about 4 inches. I have two 100mm lenses. The Meyer Trioplan 100/2.8 is a plain long lens, about 3 1/2 inches long. The Enna Sandmar 100/4.5 is a short telephoto, only 1 3/4 inches long. I suspect that internally it is a ~72/2.8 lens with a 1.4x TC optical group in back. When you add a TC to a lens, you are similarly 'shortening' the resultant lens. An actual 400mm long lens is over a foot long! A 2x TC on a short 200mm prime is half that length. But it is slow...

As mentioned above, changing sensor or film size does not change the focal length of a lens. A 100mm lens is still a 100mm lens whether it sits on a 4x5 view camera or a 110 Instamatic or anything in-between. It's just that smaller sensor or film frames see less of the projected image -- the area it sees is called the "field of view" (FOV). A 100mm lens on a full-frame camera has a wider FOV than the same lens on a crop-sensor camera like most dSLRs, but the optics remain the same.

Here's how this works. Cut out a picture from a magazine. Draw a rectangle on it that is 60x45mm - that's about the size of frame on many medium-format (MF) cameras. Inside that, draw a 36x24mm rectangle - that's the size of a full-frame (FF) 35mm camera frame. Now, inside that, draw a 24x18mm rectangle - that's the size of a 35mm half-frame (HF) or APS-C dSLR camera frame. And, just for LULZ, inside that you can draw a 18x12mm rectangle - that's the frame size of m4/3 (Panasonic, Olympus) cameras. In each case, THE PICTURE REMAINS THE SAME. The focal length and other optical characteristics remain the same. But smaller sensors see less of the picture, that's all.
12-02-2010, 01:53 AM   #14
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QuoteOriginally posted by RioRico Quote
Here is the simple, basic equation: SPEED = MONEY

The faster you want a lens to be, the more you pay for it. And the less you pay, the slower it will be, usually. A telextender / teleconverter / TC is a cheap way to get a longer lens, but the cost is speed.

Keep in mind that all telephoto lenses have a TC built into them. This "telephoto group" makes the lens physically shorter than its actual focal length. For instance, 100mm is about 4 inches. I have two 100mm lenses. The Meyer Trioplan 100/2.8 is a plain long lens, about 3 1/2 inches long. The Enna Sandmar 100/4.5 is a short telephoto, only 1 3/4 inches long. I suspect that internally it is a ~72/2.8 lens with a 1.4x TC optical group in back. When you add a TC to a lens, you are similarly 'shortening' the resultant lens. An actual 400mm long lens is over a foot long! A 2x TC on a short 200mm prime is half that length. But it is slow...

As mentioned above, changing sensor or film size does not change the focal length of a lens. A 100mm lens is still a 100mm lens whether it sits on a 4x5 view camera or a 110 Instamatic or anything in-between. It's just that smaller sensor or film frames see less of the projected image -- the area it sees is called the "field of view" (FOV). A 100mm lens on a full-frame camera has a wider FOV than the same lens on a crop-sensor camera like most dSLRs, but the optics remain the same.

Here's how this works. Cut out a picture from a magazine. Draw a rectangle on it that is 60x45mm - that's about the size of frame on many medium-format (MF) cameras. Inside that, draw a 36x24mm rectangle - that's the size of a full-frame (FF) 35mm camera frame. Now, inside that, draw a 24x18mm rectangle - that's the size of a 35mm half-frame (HF) or APS-C dSLR camera frame. And, just for LULZ, inside that you can draw a 18x12mm rectangle - that's the frame size of m4/3 (Panasonic, Olympus) cameras. In each case, THE PICTURE REMAINS THE SAME. The focal length and other optical characteristics remain the same. But smaller sensors see less of the picture, that's all.
Great explanation, and I never knew that telephotos could work like that. Learn something new everyday, at any hour...
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