I don't understand lenses

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Fancythat

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I am new to photography and find reading technical guides difficult to understand. I don't understand all the numbers that go with lenses. Can anyone enlighten me? I did look for a thread with the search feature but couldn't find anything. If there is one, can you point me in the right direction?
Thank you very much
 
You will come accross 2 sets of numbers usually, the first will be the focal length measured in mm. This might be 18-55mm, 70-300mm, 10-20mm anything really. You can also get fixed focal lengths like 35mm, 50mm, 100mm.

The Focal length is simply the view the lens gives you.The smaller the number, the more view you get into a photo, the bigger the number, the more zoomed in you get, so a smaller view.

The next numbers will be the aperture. This is the hole in the lens that lets the light in.
This is usually written like this
1:3.5-5.6

Looks complicated, but it isnt. Ignore the 1:
The next is 3.5. This means the lens at its widest setting (so 18mm on a 18-55mm lens) the aperture can only go as low as f/3.5. As you zoom the lens in further, the aperture (the hole) narrows.
Once you hit 55mm, the widest the aperture can go is f/5.6.
Of course at any time you can go higher than those numbers, so in most cases up to f/22, but not lower.
More expensive lenses have fixed apertures. This will be displayed as 1:1.8 or 1:2.8

This simply means throughout the focal length, the aperture can always be f/1.8 or f/2.8. Again you can always go higher.
 
My internet is running slow so as I was trying to post this Tom pretty much said it all but as I went to the effort of wrestling with the net I've posted anyway just in case there is anything not mentioned in Tom's post...

Well, here's a brief go at explaining some of it....

There's zoom/focal length and aperture to start with so for example... 17-50mm f3.5-5.6. Here we have a zoom lens which at the widest end (giving the widest field of view) is 17mm and at the longest zoom is 50mm. The aperture (the hole that lets the light in) is expressed as a f number and the smaller the number the bigger the aperture. At the widest end the biggest aperture we can get with this lens is f3.5 and at the longest zoom end the widest aperture we can get is f5.6.

Simples? I hope so... :D and moving on...

Some lenses can maintain a constant aperture throughout their zoom range, for example... 17-50mm f2.8. As only one aperture value is quoted here rather than something like f3.5-5.6 we can assume it's a constant aperture lens, but of course the user can still select any aperture that the lens can use from f2.8 to something like f16 throughout the zoom range. Some lenses don't zoom and these are called prime lenses, eg. 50mm f1.4.

You also get all sorts of letters and numbers tagged onto lens descriptions and it does get confusing as manufacturers use their own systems. eg. Canon EF-S 17-85mm f4-5.6 IS USM. Here the lens has image stabilisation (IS) and a USM focus motor.

I hope that helps. The best thing is to try and work it out but until you get the hang of it just ask if you get stuck.
 
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Try googling for Aperture and Focal Length.

But just as an example to help:



1: Canon 20-70mm IS f3.5-5.6 Macro

The above lens does not exist. But say it did, it would mean it was manufactured by Canon, has a zoom range of 20mm to 70mm, has image stabilisation (to assist with camera shake) has an aperture of f3.5-5.6 and is a macro version lens as well (can focus very close to things).



2: Another example:

Canon 50mm f/1.8

Canon, 50mm fixed focal length and a maximum aperture of f/1.8




3: Another:

Sigma 18-50mm f2.8

Sigma, 18-50mm focal length and constant maximum possible aperture throughout the zoom range of up to f2.8.
 
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This might help you.
 
Fancythat said:
I am new to photography and find reading technical guides difficult to understand. I don't understand all the numbers that go with lenses. Can anyone enlighten me? I did look for a thread with the search feature but couldn't find anything. If there is one, can you point me in the right direction?
Thank you very much
Click to expand...

I'm a beginner too so I understand what your going through. I kept trying to memorize the numbers and it wasn't till I stopped doing that when it all came together. Well not all (still learning). This is what I did, think of apiture as the size of the opening in your lens. The bigger it is the more light that will come in. The smaller the less light. That's it! Don't worry about depth of field just yet. That will fall into place once you get a feel for how the apiture is effecting the brightness/darkness of you exposure. I apologize to the experienced photographers for posting with such little knowledge. This is just how it came together for me. Hope it helps.
 
Oh sorry I almost forgot. Once you understand apiture in this manner the lens f-numbers become much easier to understand on lenses. Then all you need to do is understand cropping if your not using a full frame camera like me. Let me know if you get that down so you can explain it to me. Lol
 
I may be wrong but I understand the crop factor of a non-full frame sensor to mean that the focal length of the lens is essentially multiplied by 1.6.
So using a 50mm lens on a full frame camera gives you a 50mm focal length, where on a crop camera that same lens is giving you an 80mm focal length
 
beesee said:
I may be wrong but I understand the crop factor of a non-full frame sensor to mean that the focal length of the lens is essentially multiplied by 1.6.
So using a 50mm lens on a full frame camera gives you a 50mm focal length, where on a crop camera that same lens is giving you an 80mm focal length
Click to expand...

Actually no, the focal length stays the same, a 50mm lens is a 50mm no matter what camera it is on. It is the angle of view that changes when put on a smaller sensor camera.:)
 
craftysnapper said:
Actually no, the focal length stays the same, a 50mm lens is a 50mm no matter what camera it is on. It is the angle of view that changes when put on a smaller sensor camera.:)
Click to expand...

And this new angle of view would be the same as a focal length lens of 1.6x the length of the one you are using if it were on a full frame camera. :)

(1.6 if on a Canon crop, 1.4 Nikon)
 
DorsetDude said:
And this new angle of view would be the same as a focal length lens of 1.6x the length of the one you are using if it were on a full frame camera. :)

(1.6 if on a Canon crop, 1.4 Nikon)
Click to expand...

The angle of view is something that's easy to understand for newbies, however, there are more factors which depend on focal length than angle of view.

This is why experienced photographers tend to be pedantic on the issue, so that you don't have to learn twice.

So 50mm is always 50mm, even though it's a wide angle on Medium format, a standard lens on FF DSLR (or 35mm film) a short tele on a crop camera and a longer tele on a compact.
 
Phil V said:
The angle of view is something that's easy to understand for newbies, however, there are more factors which depend on focal length than angle of view.

This is why experienced photographers tend to be pedantic on the issue, so that you don't have to learn twice.

So 50mm is always 50mm, even though it's a wide angle on Medium format, a standard lens on FF DSLR (or 35mm film) a short tele on a crop camera and a longer tele on a compact.
Click to expand...

For some reason I'm having a hard time grasping this. (Crop factor and its importance) I'm sure it will come in time. Thanks.
 
shaylou said:
For some reason I'm having a hard time grasping this. (Crop factor and its importance) I'm sure it will come in time. Thanks.
Click to expand...
You do not really need to understand crop factor, it's a hangup from the days of film - for those who only ever shot 35mm film:D.

They have this unshaking belief that all photographic equipment can only be measured by it's relationship to a 35mm film frame. :thinking:Whereas those of us who have used medium format or large format equipment, or that are just happy with our sensor sizes, don't even consider it.

So my std zoom is 17-55, but that doesn't really mean it's a 28-90 equivalent - it just means that it's the right focal length for a standard zoom for me for my camera. In the same way that my 75mm was the standard lens on my MF camera - it wasn't a 50mm equivalent - it was simply the standard lens.

The one thing you need to bear in mind is that the larger the medium (whether a digital sensor, roll of film or sheet of film) the better control you can gain over depth of field.

So there's something about the 'look' of a FF image that's superior to a crop image, but that shouldn't lead us to think that FF is The standard. Because bigger is better - so MF digital will be better still and Large Format better yet.
 
Thank you very much for all your help everyone, I think it is a little clearer. I did understand aperture and the higher the number the less light, but was struggling with the focal length and how it affects aperture. I think I need to have a look through different lenses and see the differences for myself (I am a bit of a 'I see, I do, I understand' type of learner). I have bookmarked the different sources of info too, and got an invite to a local group!!
 
Just to explain one thing about the Exposure Triangle that is confusing - lens f/numbers.

The bigger the f/number, the smaller the physical size of the aperture. That much is easy enough to grasp, even if it appears backwards.

The confusing bit is that f/numbers change by a factor of 1.4x (square root of 2) because that's how the maths works, but every time you multiply/divide by 1.4, the aperture actually doubles/halves in size. So for example, f/2.8 passes twice as much light as f/4, which is twice f/5.6 and so on.

This then relates to the other two corners of the triangle, that work on a more logical linear doubling/halving scale. Eg, 1/250sec is obviously half the duration of 1/125sec, and ISO400 is twice as sensitive to light as ISO200.

In this way, you mix and match different shutter speeds with f/numbers and ISO sensitivity to deliver the corect exposure to the sensor, while getting the settings you need for pictorial effect.

If you can get your head around this, you've cracked it: settings of 1/60sec at f/11 and ISO800, will give the same correct exposure as 1/125sec at f/2.8 and ISO100. The main difference is that lens aperture has changed a lot, meaning that at f/11 you will have a lot of depth of field to get everything sharp over a greater distance (landscape?) and at f/2.8 DoF will be very shallow, putting the background out of focus and emphasising the main subject more (portrait?).
 
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