You are always using the manufactuer's original lens hood but get lens flares in the image? 

That may have two reasons.

 

1. You are using a full frame lens with a body with maller sensor. 

So to speak you have an EF lens and a body with EF-S mount or an FX lens using with a DX body (Other manufacturers use their respective acronyms). 

Vollformat vs. Cropformat

 

2. You are using a zoom lens

With this brief article I do not want to explain anything difficult but just show how different the usual formats of films and sensors are. These are the most common ones. 

common formats

Legend

  Format Measures in mm Factor
1 Medium Format 6x7 60x70 0,47
2 Medium Format 4,5x6 45x60 0,57
3 Digital MF 36x46 0,74
4 Small Film 1) 24x36 1
5 DX 15.6x23.7  1.5
6 APS-C 2) 14.8x22.2  1.62
7 Foveon 13.8x20.7   
8 M4/3 3) 13x17.3  2
9 1" 8.8x13.2  2.7
10 2/3" 6.6x8.8  3.92
11 1/1.8" 5.4x7.2  4.81
12 1/2.5" 4.3x5.8  6
13 1/2.7" 4x5.4  6.43
14 1/3.2" 3.4x4.5  7.67

 

1) Small film relates to the standard 24x36 on 135 type film. On film there other rather exotic formats too, such as 57x24 for panoramic formats, half format of 18x24 or even more exotic ones. For DSLR we usually refer to the full frame (full format) FF. 

2) APS-C is a film format which varies slightly from the quasi APS-C that Canon and Nikon uses. The difference to Canon APS-C is so small that I did not find it of being much of a value to include the real film APS-C in the graphic. 

3) M4/3 ist an equivalent of Micro Four Thirds. This is the same sensor size as FT (Four Thirds) and happens to be the same frame size as used on 110 film. 

Did you ever wonder why some images have their sharpness exactly there where the photographer wants you to look at while in others everything appears sharp?

 

 

The depth of field is that range in the room in front of the lens, that appears sharp in the image. The lens is focussed to one distinct distance in which everything is projected sharp on the sensor or film.

Well, some range in front of this focal distance is still sharp enough to appear sharp, so is some range behind the focal distance. (very roughly 1/3rd before the focal distance and 2/3rd behind) This range is the depth of field.

There are two factors influencing the depth of field.

Is the depth of field really different when using cameras with different size of their sensors? You should think it is literally just a cropped image. How could that possibly affect the sharpness?

The image left shows the ratio of the size of a crop (here canon APS-C size) compared to so called Full Frame size bodies. 

With some rather simple experiment I just checked how the sensor's size influences the depth of field. I photographed some standard ruler with two different bodies and with different focal length so that both images are taken with roughly the same angle. 

Diffraction

The diffraction is a physical phenomenon which appears when light goes along some edge. It is well demonstrated on a slit or pinhole.

We as photographers see this effect when ever we close the aperture. Then the aperture acts similar to the pinhole in this article on Wikipedia.

Diffraction in Photography

The light comes into the lens and passes the aperture on its way to the film or sensor. It is either blocked by the aperture or passes through the hole that the aperture forms. That part of the light that passes the edge of the aperture very closely is affected by the diffraction. The smaller the aperture is the larger is the portion of the light that is affected.

Stars