Wide aperture - Pictures out of focus

Started Aug 3, 2014 | Discussions thread
Truthiness Regular Member • Posts: 122
Re: Less than $300.00.....

tedolf wrote:

Truthiness wrote:

tedolf wrote:

Thus, before buying a fancy f/1.8 85mm portrait lens you would think that a person would look at a DOF table and say, "hey, this thing isn't going to work for tight head and shoulder shots wide open-the DOF is too shallow, maybe I would be better off buying an 85mm f.2.8 lens instead and save a few hundred dollars!".

Would you point out to me how many 85/2.8 lenses there are on the market which cost about 100 dollars or 200 dollars?



Less than $300.0 and that is new. Used it is less than $200.00

I wonder if any other system has one slow 85...

Canon 85/1.8 is less than 400 bucks new, so still only a 100 buck difference.


So now you are so desperate yuo try to find old manual focus lense you need adapter for. You know there are 85/1.8 manual focus lenses which don't cost much at all? You're getting a bit silly now.



There are many others you can find, I will leave the exercise for the reader.

Jupiter-9 85mm f/2 - a very popular lens, about 100 bucks. Now you need to find a slower lens which is on the market for negative amount of money.

85mm f/1.8 lenses from Canon and Nikon are inexpensive yet optically very good. The CaNikon lenses in question cost 400-500 dollars.

The m43 45mm f/1.8, a two stops slower lens that the above FF lenses, costs 350 bucks.

It is exactly the same maximum aperture, not two stops "slower".

f=focal length. the f-stop numer forms a formula where focal length is divided by a number.

Thus 85/1.8 has entrace pupil of 47,2mm, while (the little more narrow or long) 45mm /1.8 lens has entrance pupil of 25mm.

The maximum apertures are two stops apart (considering the slightly different field of view).

The aperture number is different from aperture.

Please think of the word speed. Why it is commonly used? Isn't it because a fast lens allows for faster shutter speed for the level of image quality required?

So if on m43 f/2 gives image quality of x, on FF f/4 gives the same image quality x. Thus f/2 on FF is two stops faster.

Please, F-stops are the mathematical ratio of focal length to lens diameter.

No, it's a ratio of focal length to entrance pupil. Have you noticed that 45mm lens and 85mm lens have a different focal length, thus the aperture diameters are different for the same f-stop?

The 42.5/1.2 Leica/Panasonic is still a stop slower,

No, it is one stop faster.

No, one stop slower. A 85mm FF lens at f/2.4 allows identical amount of light to pass the m43 Leica does at f/1.2.

Just above you understood that f/# is a ratio - why not calculate how much 85/1.8 is and how much 45/1.2 is?

Let's get back on topic. We are talking specifically about the image the OP tried to take.

We can go back to that, but not until you see the light

To still use f/1.8 and......oh, forget it.

You don't need any more ND filters on FF f/1.8 than on m43 f/1.8. Light per area is the same, light per photo differs by factor of four.

forget it, I don't want to explain it to you. It is just going to get you off on a tangent.

The f-number tells us about the density of light hitting the sensor. The FF and m43 have similar capability of holding the light without saturation per area. FF has about 4 tiimes more area and it in this case collects 4 times more light. Thus ND is irrelevant to the topic.

An example. Both sensors have 16MP, the FF pixels have about 4 times as large full well capacity (FWC). At f/1.8 if both sensor have the same exposure time, the FF collects about four time as much light. But because it's collection capacity (FWC) is also four times larger, it'll manage just fine without ND filters.

The problem is that you don't understand the very basics here. I'm not trying to insult you or bash you system, but just to teach you a bit. I would not do that, but since you started writing factually false stuff, someone needs to correct it so that the OP et al won't misunderstand the reality and maybe buy a camera based on incorrect information.

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