Dumbest Question of the Year - and year is just starting!

wildviper · February 6, 2009

Well, here goes.

I have the Sigma 70-300/4-5.6 APO Macro lens with the red line. I am playing around with the aperture ring on the lens(disconnected from camera) and see that when I zoom out to 300mm, I can still get the aperture to f/4. I look through the lens and see the opening fine. In fact, I can zoom in and out all day and the f/4 will be set with no problem.

So, naturally, the cheapo that I am, I started wondering if there is a hack to over-ride the camera communication with the lens for the aperture setting????

This way, I will connect up my lens back to my D70s, set the lens on F/4 and hope that I get a "fast" lens at 300mm.

Told you this could be a dumb question. To my "don't know a lot about photography" mind, this seems plausible. :scratch

So what say you?

frel · February 6, 2009

wildviper wrote:

Well, here goes.

I have the Sigma 70-300/4-5.6 APO Macro lens with the red line. I am playing around with the aperture ring on the lens(disconnected from camera) and see that when I zoom out to 300mm, I can still get the aperture to f/4. I look through the lens and see the opening fine. In fact, I can zoom in and out all day and the f/4 will be set with no problem.

So, naturally, the cheapo that I am, I started wondering if there is a hack to over-ride the camera communication with the lens for the aperture setting????

This way, I will connect up my lens back to my D70s, set the lens on F/4 and hope that I get a "fast" lens at 300mm.

Told you this could be a dumb question. To my "don't know a lot about photography" mind, this seems plausible.

So what say you?

The fact that the aperture does not change as you zoom is indeed a proof that your lens is a variable aperture.

Let me try to explain : the longer your zoom is -- more precisely the further your iris is from the end of the lens -- the larger it has to be opened to keep an equivalent aperture. Basically as your lens is now longer this iris as to be more opened to let enter the same amount of light. This require the fixed aperture lens to have in fact a complex mechanism (implying part of the price) that open/close slightly the iris as you zoom in/out. The benefit is that your aperture remain constant in your zoom. And actually if you were able to deactivate that variability in the iris you will be probably able to gain some aperture at your wider zoom but would also loose this independence of aperture to zoom.

Getting of this system makes lens design easier and allow people to benefit to the wider iris aperture at the wider zoom (allowing zen people to have a f/2.8-x lens for cheapo) but then as they zoom in/out they do not have the same aperture (understand the same amount of light entering) which can be problematic in certain condition (external flash is what comes to my mind at first, but more generally the fact that you need to adapt your shutter speed as you change the zoom). Your camera is aware that the aperture change and can adapt to that.

Hope it helped you understand more

wildviper · February 8, 2009

Ahh..I got it now! Thanx for that explanation. I knew this would not be possible.

Candid Arts · February 8, 2009

Too add on to that, this equation:

take your focal length, and divide it by the aperture, and you get the physical opening size of the aperture. For EX:

100mm lens at f/4 would give you a 25mm aperture opening (100/4=25)
200mm lens at f/8 would give you a 25mm aperture opening (200/8=25)

Note that you zoomed in from 100 to 200, the variable aperture changed from f/4 to f/8, yet the actual size of the aperture opening stays the exact same.

Hope that helped.

JustPlainMe · February 8, 2009

Candid Arts wrote:

Too add on to that, this equation:

take your focal length, and divide it by the aperture, and you get the physical opening size of the aperture. For EX:

100mm lens at f/4 would give you a 25mm aperture opening (100/4=25)
200mm lens at f/8 would give you a 25mm aperture opening (200/8=25)

Note that you zoomed in from 100 to 200, the variable aperture changed from f/4 to f/8, yet the actual size of the aperture opening stays the exact same.

Hope that helped.

Wow, I was just browsing the threads and this is the best explanation of the relationship betwen focal length and aperture I've seen. I love math, but my eyes get all blurry when I try to figure it out. So when you stop down at a longer focal length but keep the ratio the same, shutter speed is the only variable that will affect how much light gets in? This makes a lot of things make much more sense. Thank you!!

Sarah

Candid Arts · February 8, 2009

JustPlainMe wrote:

Wow, I was just browsing the threads and this is the best explanation of the relationship betwen focal length and aperture I've seen. I love math, but my eyes get all blurry when I try to figure it out. So when you stop down at a longer focal length but keep the ratio the same, shutter speed is the only variable that will affect how much light gets in? This makes a lot of things make much more sense. Thank you!!

Sarah

Well I'm glad it helped. But don't forget about ISO as well. Although technically ISO doesn't change the amount of light coming in to the sensor, but how sensitive the sensor is to light, there fore affecting how much light is needed to properly expose.

JustPlainMe · February 9, 2009

Candid Arts wrote:

Well I'm glad it helped. But don't forget about ISO as well. Although technically ISO doesn't change the amount of light coming in to the sensor, but how sensitive the sensor is to light, there fore affecting how much light is needed to properly expose.

Right, but I'm thinking strictly in terms of how wide the opening is and how long the light will be let in. It will help me understand why my camera chooses certain shutter speeds in aperture priority mode at particular focal lengths, and learn how to dial in the right settings on my own in manual mode. Then I can adjust ISO manually if need be, and maybe eliminate some of the frustration of the learning curve.

Sarah

Candid Arts · February 9, 2009

JustPlainMe wrote:

Right, but I'm thinking strictly in terms of how wide the opening is and how long the light will be let in. It will help me understand why my camera chooses certain shutter speeds in aperture priority mode at particular focal lengths, and learn how to dial in the right settings on my own in manual mode. Then I can adjust ISO manually if need be, and maybe eliminate some of the frustration of the learning curve.

Sarah

Well excellent. Glad I could help!

Good luck with your photography.

Dumbest Question of the Year - and year is just starting!

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