Flash exposure compensation with 2X teleconverter on M645

[chad_worthman1]

Hi there,

 

I've got a Mamiya M645 with 80mm 2.8 lens and a Rokunar 2X MC6

teleconverter. The previous owner told me that this 2X teleconverter

has a 2 stop effect on the exposure.

 

When using the teleconverter without flash, my metered prism is TTL,

so I don't have to worry about exposure compensation, correct?

 

What about when I'm using flash? I'm using a Nikon SB-26 mounted on

the hot shoe in Auto mode, I enter my film speed the same as the

meter, but do I manually boost the f-stop on the flash?

 

For example, say a shot requires F/5.6 @ 1/60 secs with the 2X

teleconverter on the lens. Do I set F/11 (2 stops more light) on the

flash?

 

Thanks,

 

Chad

[jeg]

Teleconverter basics: a 2x TC changes your maximum aperture (wide open) to two stops less than the lens has without the TC. The 80mm f/2.8 becomes a 160mm f/5.6 lens. With the TC attached, you can set an aperture of f/2.8 or f/4, but you will still take pictures like at f/5.6 (means underexposure if you non-TTL meter at f/2.8 or f/4). So everything from f/5.6 on should be exposed regularly as f/5.6.

 

As the 160mm focal length is a bit longish, I would add a +1/2 or 2/3 stop compensation when making pictures with it (and without TTL metering). Trust me.

[jeg]

Small correction:<blockquote>

"So everything from f/5.6 on should be exposed regularly as f/5.6 <i> or as whatever aperture you are using (f/8, f/11, ...)</I>."

</blockquote>

[chad_worthman1]

Thanks,

 

That's different than I expected. I thought all apertures would be effectively 2 stops slower. So you're saying that when you attach a 2X teleconverter to a lens with the aperture range of 2.8 - 22, it becomes a 5.6 - 22 and not a 5.6 - 45?

[bueh]

Of course it becomes a 5.6-22. Or do you see markings for f/32 and f/45?

[chad_worthman1]

There are no markings for 32 and 45, but that's what I thought the effective aperture would be, f/22 on the lens, with 2 extra stops for the teleconverter, makes it effectively f/45.

 

Internally to the lens, the aperture is the same size for f/22 (without TC) and f/45 (with TC), but the teleconverter only lets 1/4 the light through, so the /f22 aperture is effectively f/45.

 

The lens marking says it's still f/22, but you still have to lengthen the shutter 2 stops.

 

Does that make sense?

[bueh]

No Chad, it doesn't. Again, as JEG said, the lens becomes a 160mm f/5.6 lens. It doesn't matter if you select f/2.8, f/4 or f/5.6, the effective aperture is always f/5.6. You don't add two stops for the TC, you simply lose two stops at the wide open end. It's not because the TC eats two stops of light, but because the focal length of the lens is now much longer than before and also because the TC pretty narrow. f/8 is still f/8 with or without the TC -- the lens opening is small enough not to be blocked by the TC. And the TC is a kind of magnifier, which means while you have the same effective aperture size at f/22 with or without the TC, it will magnify it so it becomes f/22 at 160mm. By the way, your lens with the TC at f/22 will have the depth of field of an 80mm at f/45 (hint: it's pretty much the same).

<p>

Don't worry, just take pictures with it. Meter normally, add a little compensation for the TC and everything will be fine.

[chad_worthman1]

OK, I got it now.

 

Thanks very much,

 

Chad

[simon_platt1]

Chad

I believe the answers you have been given are incorrect.

 

Your original assuption is spot on.

 

Without a teleconvrtor you have a f2.8 to f22 lens and with your telecovertor it becomes f5.6 to f45.

There are a number of other posts on this and the other 35mm forums to this effect and here is why.

 

Aperture value relates the diameter of the iris and the focal length of the lens.

 

So f2 on a 100mm lens the apparrent aperture diameter will be 50mm.(Often some adjustment is made to account for light loss in the elements so the actual size may be sligtly different)

 

The aperture is mechanically set by the aperture ring. So if you set it to a size that gives F5.6 at 80mm. If the lens is now altered with a TC to become a 160mm then as the aperture is still the same size as originally it must now be F11.

 

I don't have a Mamiya to test hoever it is possible that there is an internal mechanism which alters the mechanical size of the aperture when the teleconvertor is attached.

 

The point about some extra compensation is good because there will be a light loss in passingthrough the teleconvertore that may well mean that the actual adjustment should be 2 and 2/3 stops

 

Some Nikon teleconevrtors are marked with an aleternative aperture ring to be read instead of the normal one when the TC is in use.

 

I suggest that you prove this for yourself. Get a roll of slide film and set your flash to an auto aperture of F5.6 or F8 then shoot the whole roll starting with the lens set to F2.8 and reducing it in half stop intervals. Make notes so you can relate flash setting to aperture used for every frame. When you getthe film back look carfeully at the frames and see which one looks correct to you. the difference between this and the flash setting is the one to use.

 

TTL metering of ambient and flash along with electronic aperture control avoid all of these calculations.

[bueh]

Yeah, Chad, waste a roll of film and find out that my advise is correct. Simon is forgetting about the magnification effect of the TC, so while the physical aperture might be the same (means it's "smaller" for the longer focal length), the effective aperture is twice as large (just like the focal length) which means two stops more light. f/22 is f/22, not f/45. Only wide open the diameter of the TC limits your maximum aperture, that's why you lose those two stops.

[chad_worthman1]

I did a little testing last night and it appears that if I use the TC, and open the lens wide open (2.8) , then close to 4 and 5.6 the meter responds each time, which contradicts what Bueh is telling me and supports my original assumption. If what you're saying is correct Bueh, wouldn't the meter keep the same value from 2.8 to 5.6?

 

If you use extension tubes, there's an exposure compensation factor because of the extra distance from the front element to the film plane. I thought it would be the same for the TC. I can't see how a TC can avoid that effect.

[bueh]

Chad, I agree that this is strange. Maybe that Rokunar TC uses an alternative aperture set or a more sophisticated coupling mechanism that alters the lens' aperture. Normally this shouldn't happen. But if your TTL meter says so, I would rather trust it. It would be interesting to compare exposure data for each shot with and without the teleconverter at the same aperture setting and compare it to what a hand-held meter gives you. Take notes and learn to use the equipment at hand correctly. But don't forget that the M645 prisms aren't that accurate, though I found them sufficient most of the time. Good luck and post your results!

[q.g._de_bakker]

Bueh,<br><br>You always lose two stops. The 'effective aperture' is always two stops smaller. Not just wide open.<br>The best way of understanding how teleconverters work is by keeping in mind that they do not change the focal length at all. After the lens has produced an image, they magnify that image 2x. They do that by spreading the image out; 2x larger in all directions. That means the area the light coming through the lens is spread over increases by a factor 4, the illumination decreases by the same factor, hence a 2 stop los in light.<br><br>Always.<br><br>f/x might be f/x, but the image is two stops dimmer.<br><br>Always.

[q.g._de_bakker]

Chad,<br><br>The effect is indeed very similar to what happens when using extension.<br>When using extension, the exit pupil is moved away from the film, causing a larger, 'thinner' spread of light. When using a converter, the image is spread out by the optics of the converter.

[simon_platt1]

Chad

As I said the best way to learn is do you own tests. Prove to yourself exactly what happens to your camera. Compared to the cost of spoiling pictures using up one reel of film is nothing.

 

Bueh and JE G

 

I would like to quote from;

 

Focal Press Applied Photographic Optics 2nd edition page 400

 

'Along with the increase in Effective Focal Length(EFL) is a loss of aperture related to power. The entrance pupil is unchanged in size and position and so a doubling of EFL also doubles the F number'.

 

It also goes on to mention that the teleconvertor system was used in a number of telephoto lens designs.

 

I didn't forget the magnifaction effect but it is this very magnification that causes the reduction in level. The same amount of light is being spread out over a large area.

 

If you think of light as bundles of energy it might help. The aperture has remained constant so a fixed amount of light energy has got through it. You cannot add anything to this energy so when you magnify up the central portion of the image you spread the energy over a larger area and so the net amount per square mm goes down and the image is dimmer.

[scott_sharp]

I just recieved the Mamiya Tele-Converter 2X N with the original box and instructions. I'll post some of the information

from the instruction sheet.

 

Automatic Photometry:

When using either of the AE Prism Finder N, AE prism Finder FE401, there is no need of exposure compensation,

since its metering system automatically functions. When an independent exposure meter has been used,

photograph after opening the just metered aperture by two stops.

 

Flash Photography:

When auto flash photography is used, or calculation is based on the guide number, use an aperture two stops

lighter than the set f value.

 

Focal Length and Intensity of Light: (Table) Master Lens focal length listed first with the Effective Change in Focal

Length and Aperture listed second:

1) 150mm f/2.8 converts to 300mm f/5.6 2) 150mm f/3.5 N converts to 300mm f/7 3) 150mm f/3.8 N/L converts to

300mm f/7.6 4) 200mm f/2.8 Apo converts to 400mm f/5.6 5) 210mm f/4 N converts to 420mm f/8 6) 300mm

f/2.8 Apo converts to 600mm f/5.6 7) 300mm f/5.6 N converts to 600mm f/11 8) 500mm f/4.5 converts to 1000mm

f/9 9) 500mm f/5.6 converts to 1000mm f/11