 Morris Turner |
Teleconverters Gonna ask a silly question... is there a formula in figuring f-stops when TC's are used? For example, you've a lens with f 4.5 and you decide to add a 1.4X TC? How's it figured? Thanks for your help.
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Jon Close |
Multiply the lens's indicated aperture by the multiplication factor of the teleconverter. In your example, the effective aperture is f/4.5 x 1.4 = f/6.3. In terms of "stops," 1.4x is one stop, 2x is 2 stops.
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Morris Turner |
Thank you so much for your expertise, Jon.
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Peter Blaise Monahon |
Yeah, but the formula? ;-) For 1.4x = -1 f/stop 2x = -2 f/stops 3x = ... -3 f/stops? ... and what about those lovely 1.5x and 1.7x teleconverters? Although I can "approximate", and with TTL meters it doesn't matter since the meter will read the available light through the lens/teleconverter setup ... ... has anyone seen a formula where teleconverter strength number x C = -change in f/stops? Thanks Peter Blaise
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Steven Chaitoff |
Peter, a 3x teleconverter doesn't mean a 3f/stop change. You can't say TC strength x C = change in f/stops because that would mean TC increases with f/stop change linearly. And this is definitely not the case because what an f/stop means is a halving or doubling of the amount of light...that is, it goes up or down exponentially. So in order for f/stops to go up one by one as you increase the TC strength, the TC strength must increase exponentially by some factor C as you said. From that pattern you started above, you can see that the number of f/stops goes up faster than the teleconversion factor. Specifically, the f/stops go up by 1 f/stop each time. Meanwhile, the teleconverter sizes go up by 1.4 each time. (Or actually the square root of 2 to be exact.) Anyway, since it's NOT a linear progression, to derive an equation from that you have 1.4x = y or in other terms, y (the f/stops) equals 1.4 raised to the teleconverter strength power. Now keep in mind the f/stop answer you get is not actually in f/stops. It's a factor of change in the f/stop. 1.414TC=f/stop factor So if you have a 1.7x teleconverter on a lens with a Max Aperture of 2.8, the new aperture is: 1.41.7 = 1.77 change factor. Then multiply that times 2.8 to get the new max aperture of roughly 5 stops. About in between 4 and 5.6 stops. Now say you want to go the other way and find out what TC magnification to use at a certain aperture. For example, say you can't autofocus past f/8, and you'r Max. aperture is f/2. The equation is 1.4TC = (8/2) so simplify and you have TC log1.4 = log4 so TC = log4 / log1.4 which conveniently equals 4. So the largest TC you can autofocus with using that lens and camera is a 4x TeleConverter. In the same fashion, the -3 f/stops change you guessed above doesn't equal 3, but rather log3 / log1.4 = 3.17. Whew...and you thought you'd never use logarithms again.
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Benjamin Rivera |
So, if I have TTL metering and TTL flash metering (I have a Praktica BX20S) do I need to do anything except attach the teleconverter and shoot? What about on my MTL3? Also, I want to use an adapter to connect my MTL3 (M42) lenses to my bayonet camera (BX20S). Do I need to do anything special here? Someone said I had to "stop down" (no idea what that means) and someone else said the camera body would take care of the necessary adjustments. Help!
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Jon Close |
With through-the-lens metering you do not have to make any exposure adjustment for the teleconverter. The meter already "sees" the reduced light transmitted. When you use M42 screw mount lenses with an adaptor on your bayonet mount camera, there is no linkage of the aperture control to the camera. So you have to use stop-down metering:
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Benjamin Rivera |
Thanks for your response. Why is there no linkage of the aperture control if there is an electrical contact on the adapter? Also, it seems that the only difference in 1-4 from what I normally do is I have to focus with the lens open instead of at the aperture I will be shooting with. Is that right? Will the shutter speed and aperture be any different than if I had used the lens without the adapter?
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Benjamin Rivera |
I know I asked this sort of thing already, but am hoping for some clarification. When using my adapter to use M42 lenses on my bayonet mount camera, the user's manual says I have to meter at working aperture. What does this mean? How do I determine this for different lenses? What aperture do I use when using my dedicated flash with different lenses? I thought TTL metering took care of all of this, but since it doesn't I have no clue. A step by step like the one above would be great. Thanks!
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Jon Close |
Benjamin, the 4 steps I gave above (specifically steps 2 & 3) is what the manual means by "meter at the working aperture." With a dedicated TTL flash you don't do anything different.
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Benjamin Rivera |
OK, what I usually do is select the aperture based on how much light I need to get a nice slow shutter speed (usually 1/30) and still have enough light according to the meter. I think this is basically shutter priority, right? Since this is the case, I don't determine my aperture by using my brain, just my light meter. So, if I select 1/30 let's say, can I just adjust the aperture until the light meter is OK when using the adapter? If so, that's what I always do without the adapter, so nothing would change. If not, what's different? I guess I'm just asking how this 4 step procedure is different from what one would do under normal circumstances. Thanks.
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Jon Close |
The difference is that with lenses compatible with your camera, when you select the aperture you want, the lens stays wide open. It is closed down to the set aperture by the camera when you trip the shutter. You are able to use the lens in manual, and aperture priority, and - because the camera is able to control the aperture - also in shutter priority or full program (camera chooses both shutter and aperture). With a screw-mount lens and adaptor, you cannot use shutter priority or full progam modes because the camera cannot control the aperture. When you set the aperture wanted, the lens stops down and the viewfinder goes dark making it difficult to focus. So you have to focus at full open aperture, then stop down, take the photo, then open the aperture again to focus the next shot.
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Benjamin Rivera |
Interesting... I thought when I turned the aperture ring the aperture actually changed (when using the compatible lenses). So when using the adapter can I still use "auto" mode? On my camera auto mode selects the aperture and shutter speed automatically, and then I usually change the aperture to get the shutter speed I'm looking for. If the brightness of the viewfinder is the only issue, then I guess I can trust the suggested shutter speed based on the aperture I've selected after focusing, right? Do I keep my lenses on "A" (automatic) like usual? The only thing I'm worried about the proper exposure--I understand the potential focusing trouble. Sorry to ask so many questions, it just seems so similar that I can't tell exactly what to DO differently, even though I know what the camera is doing differently. Thanks!
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Peter Blaise Monahon |
Hi fellow Better Photographers, First of all, TTL Through The lens metering obviates the need for all this below, but ... Okay, there are 2 formulae for teleconverter power to f/stop changes, and they are both simple: -------------------------- FORMULA ONE: To find the resulting f/stop of a lens with a teleconverter, multiply the f/stop setting of the lens by the teleconverter marking! Doh! Example, an f/~1.4 lens * with an ~1.4x teleconverter would equal ~1.4 x ~1.4 = ~1.96 ... that is, f/~1.96, which is effectively equal to f/2, so an f/1.4 lens with a 1.4x teleconverter makes a setup with an f/2 maximum aperture. The numbers work cleaner when you use 2^.5 or the square root of 2 (2 to the power of 1/2) which is 1.4142135623731 ... but nobody labels their lens 50mm f/1.4142135623731! ;-) So, that is ONE rule - what is the resulting f/stop. -------------------------- Now, how many f/stops away does a teleconverter push a lens? If an 1.4x teleconverter moves a lens 1 f/stop darker, and a 2x teleconverter moves a lens 2 f/stops darker ... then, as the original question went, where does a lens move to when using an inbetween teleconverter or greater, such as one of those odd 1.5x or 1.7x? There are two ways to resolve the question. One is to COUNT after multiplying, let's use that never f/1.4142135623731 x 1.7 = 2.40416305603427 ... which is 1.531070 f/stops darker. How do I know? FORMULA TWO: Alternatively, we could take a LOG base 1.4142135623731 (or LOG base square root of 2, or LOG base 2^0.5) of the teleconverter. Here, in Microsoft Excel is the formula: =LOG(TC,2^0.5) ... where TC = the teleconverter power as labeled. So, for the 1.7x teleconverter in our example, Log(1.7,14142135623731) = 1.53106949272595 Wow, that f/Calc program is quite accurate! And for the 3x teleconverter: Log(3,1.4142135623731) = 3.16992500144231 ... which equals the 3.17 that Steven found (see below and previous post). Now, it should be quite easy for you all to build a spreadsheet of the powers of your own teleconverters versus the f/stop results of your own lenses! Enjoy. * I write f/~1.4 as the math works better with f/stops NOT rounded off. Here's the sequence where each lower f/stop is the previous f/stop times the square root of two. We don't always write them this way, but here's how the computers in our cameras see our f/stops: f/ 1
PS - I found problems with Steven C. formulae as written, though I'm sure there are many ways around the topic and many resolutions that produce accurate results. 1.414^TC=f/stop factor fails for me. This is what I get: 1.414^1.41 = 1.63252691943815 BUT, a 1.4x teleconverter as in the first line does NOT produce a 1.63252691943815 f/stop change. Then, the log3 / log1.4 example says a formula would read: log TC / log square root of 2 = f/stop change And that works for me, though it's more typing in Excel, where the formula for f/stop factor by this method is: =(LOG(TC))/(LOG(2^0.5)) ... where TC = the teleconverter markings. The results agree with my example FORMULA TWO above, quoted again here for comparison: =LOG(TC,2^0.5) THANKS, STEVEN! Click! Love and hugs, Peter Blaise Monahon
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