Does anybody know about flash...

[thierrylaflamme]

Here is my question:

 

I just bought a flash to add on top of my old Pentax Asahi K 1000. I use this

camera with a zoom Sigma 75-300mm. How should I compensate for the lack of

lighting (let's say at... 200mm) with the zoom. On the back of the flash,

there are indications for the aperture that I should use on my camera versus

the distance of the subject; however, I assume these indications are based on

a regular lense (50mm), not on a zoom.

 

Thanks to clarify.

 

I usually take pictures under natural light. So, I don't know a lot about the

way it works with a flash.

[jtayloreckstein]

Does the box/instruction include any sort of indication for angle coverage? Most flashes

widest angle is 35 degrees in my experience. Many newer flashes have variable zoom

heads that can focus the light in a certain area in order to throw the light further.

 

Did it come with a zoom adapter? If it didn't, then no need to worry. Having a wide angle

coverage and using a tighter lens won't throw anything off. As long as you judge your

distance correctly. Really, the guide on the back is trying to get the right exposure, it's not

changing coverage (it can't without an external adapter or by changing the flash's

settings).

 

This is my understanding, so check me. In my opinion with flashes, experimenting with

underxposure will do wonders.

[jtayloreckstein]

It helps if you think of zoom as cropping before the film is taken. You are just magnifying a

portion of what a 50mm would see (or cropping the image).

[jimstrutz]

The problem I see is that this lens does not hold a constant aperture while zooming. So let's say you set the lens at 70mm f/4 and the flash for the same f/4 aperture, but if you then zoom to 300mm, the lens is now really at f/5.6, and the flash will be one stop darker than you want.

 

This isn't a big issue with modern TTL flash metering, but the K1000 doesn't have that. You're probably going to have compensate by making aperture adjustments as you zoom the lens out.

[jtayloreckstein]

Thanks Jim, I hadn't thought of that point.

[thierrylaflamme]

Thanks for your help! I understand better.

[gdw]

Thierry, when using flash is it strictly output of the flash unit and the distance from the flash unit to the subject that governs the f/stop.

 

The light from the flash unit will be the same at ten feet or twenty feet or whatever regardless of what focal length lens you are using. The f/stop is gong to be governed strictly by the distance of the flash from the subject.

 

When you say you have a guide on the back of the flash I am making an assumption that you are using an older using that does not have a zoom head on the flash. However, the principle is the same regardless. At a specific guide number the f/stop suggested by the guide on the back of the flash will remain the same regardless of focal length of the lens.

 

You will be limited by how far away the subject can be by the maximum output of the flash and the maximum lens opening you have available.

 

There are considerations regarding coverage. But there should not be a coverage problem in the focal length range you mention. Generally flash coverage is only an issue on lenses shorter than 50mm not longer.

 

There are considerations regarding variable aperture lenses. However, if the guide on the back of the flash says to set the aperture at f/5.6 for a subject ten feet away it would be the same regardless the focal length you are using. The focal length of the lens has no effect on the output of the flash unit. F/5.6 at 75mm is the same as f/5.6 at 300mm.

 

If you have a variable aperture lens you, one that varies the aperture and you change focal lengths, you should have some way of knowing what the adjusted f/stop is at the various focal lengths. In which case, you would want to be sure that you are going by the adjusted f/stop.

[thierrylaflamme]

Thank you so much Gary! Your help is really appreciate.

 

Greetings,

 

Thierry